Comprehensible input sounds like a fancy technical term, but what exactly is it?

According to linguist Stephen Krashen, comprehensible input is that kind of input that is "a little beyond" our current level.  This is how we acquire language and it certainly makes sense, but does it tell the whole story? 

Is comprehensible input all we need to learn languages successfully, or are we still missing other pieces of the puzzle?

Let’s find out!

What’s behind Krashen’s Comprehensible Input?

If you've never heard of Stephen Krashen before, then he's someone whose work you should really get to know. Krashen is a widely-published linguist whose ideas form the backbone of Second Language Acquisition (that is, language learning) as we know it today.

In particular, Krashen is known for his many conjectures, hypotheses, and other ideas, two of the most popular being the concept of "comprehensible input", as well as the Input Hypothesis that goes along with it.

"Comprehensible input" is input that is slightly above your current language skill. In Krashen's writings, you also see this referred to as "i + 1", where "i" is your level right now, and the "+1" refers to whatever the next level above yours might be.

With that in mind, let's now dig into the "Input Hypothesis.”

Input Hypothesis

In Krashen's own words, the Input Hypothesis suggests that:

"We acquire language in only one way: when we understand messages, when we obtain 'comprehensible input'. We acquire language, in other words, when we understand what we hear or what we read—when we understand the message."

According to Krashen, if the Input Hypothesis is true, then a few other things must also be true:

  1. "First, language acquisition is effortless. It involves no energy, no work. All an acquirer has to do is understand messages."
  2. "Second, language acquisition is involuntary. Given comprehensible input, and a lack of [emotional] barriers, language acquisition will take place. The acquirer has no choice".

If you've never heard these ideas before, then they might seem pretty shocking. It's rare to hear anyone use words like "effortless" and "involuntary" when it comes to language learning; in fact, I think we're more used to hearing words like "difficult" and "frustrating" whenever the subject comes up!

So what gives? Where is the disconnect between Krashen's theory and the apparent "reality" that so many of us language learners face every day?

To better understand where Krashen is coming from, I think it's useful to define a couple of terms. First, let's talk about explicit learning, and second, let's examine implicit learning.

"Explicit learning" is the kind of learning that takes active effort. If you studied a language in school, this is likely the type of learning that you engaged in, most of the time. It involves deliberately trying to take what you don't know and drill it into your memory.

"Implicit learning" is quite the opposite. This type of learning is done passively, usually through mere exposure to the information you are trying to learn. A good example of this is how you learned your mother tongue. When you were a child, you probably never had to do grammar drills to learn the verb tenses of your first language. It just happened naturally!

When Krashen says that comprehensible input makes language acquisition "effortless", he's saying that it provides an optimal environment for "implicit learning"—again, learning that is done passively, rather than actively. By contrast, the effortful language learning that you're used to most likely involves little comprehensible input and a lot of "explicit learning", which is by definition more difficult.

Can You Really Learn a Language Effortlessly?

While the idea of "effortless language learning" certainly sounds more appealing than the frustrating learning experiences most of us had in school, I'm not entirely convinced that an all-comprehensible-input-based, implicit learning method is the way to go. 

I think Krashen may have missed the mark here for one reason; a flaw in implicit learning that I've encountered often when trying to put it into practice myself:

The flaw? Implicit learning, though effective, is incredibly SLOW—often painfully so!

Think back to the example I gave for implicit learning: a child learning his or her first language.

Unlike adults, children have no use for explicit grammar instruction. They're able to learn their native languages perfectly, solely through absorbing Krashen's so-called "comprehensible input".

The problem, however, is that this method takes incredible amounts of such input, and takes YEARS to show meaningful results. It's easy to forget, for example, that most children don't reach a conversational level in their mother tongues until they are between three and five years old!

I don't know about you, but I don't have a way to get comprehensible input 24 hours a day, seven days a week in any of my target languages. And certainly not for YEARS. Even if I could spend my time that way, I doubt I would want to. I have a life, and other interests, after all. 

Comprehensible input is clearly useful, but wouldn't it be better if there were a way to—you know—speed things up a bit?

That's where Krashen and I diverge. I believe that there's a better way for adults to learn foreign languages—one that can enable most of us to reach a conversational level quickly, without being as effortful and frustrating as explicit language learning often can be.

While Krashen believes that knowledge gained through explicit learning (that is, explicit knowledge) can never turn into implicit knowledge, I believe that explicit knowledge can often facilitate and reinforce information gained implicitly.

Let me give you a practical example:

In Krashen's model, (technically called the "No Interface" model, since it holds that explicit knowledge does not interface with implicit knowledge) learning occurs only through comprehensible input. You expose yourself to text or audio in your target language that is calibrated to be just above your level, and then learning just takes as long as it takes.

You don't worry about things like native-language translations or grammar explanations because they can't do anything for you; they're not comprehensible input, so they're effectively useless.

In my preferred model (technically referred to as the "Weak Interface" model, since it holds that explicit knowledge can interface weakly with implicit knowledge) you would expose yourself to comprehensible text or audio in your target language in much the same way, but you would supplement that exposure with external learning aids, many of which would be in your own, native language.

These can include translations of the text you are reading, or even short grammar explanations that help you make sense of a grammar pattern in the audio you're listening to.

Learning with this "Weak Interface" model is much faster than Krashen's alternative, merely because it allows you to exert some control over the speed at which you absorb new information. You don't have to wait for your brain to magically "decipher" the meaning of a word or grammar pattern through context, but rather you're able to turn to a translation or grammar notes and discover instantly what those new things mean.

These learning aids don't try to do all the work for you (because they can't), but they can give your learning a helpful nudge when your brain isn't absorbing the new information as fast as you would like.

How to Speed Up Your Language Learning

To be clear, I'm not exactly saying that Krashen and his Input Hypothesis are wrong; comprehensible input should be the foundation of any language learning plan. All I'm saying is that it's not sufficient in and of itself to provide an efficient and enjoyable learning experience, especially for adult learners, like you and me.

I myself try to get as much comprehensible input as I can, but I always supplement that input with learning tools and aids in my native language (or a language I know well), so that I can make the learning process as efficient as possible.

Let me give you a few examples of what I'm talking about:

  • Before I read and listen to a target language dialogue, I always make sure I obtain (or create) a bilingual copy of the text (affiliate), in both my native and target languages. This gives me a way to quickly fill in gaps in understanding as I mentally process the target language content.
  • As I get my input, if I find that there's a grammar structure that I repeatedly fail to understand, I won't just gloss over it until my brain figures it out. Instead, I'll go online and read a short explanation of the grammar point in my native language, and maybe even take a short quiz on it, if one is available.
  • As my input levels increase, I don't simply let my pronunciation skills grow naturally, based upon the pronunciation of the native speakers I've listened to. Rather, I seek out feedback from others, and even do a little technical research on the sound system of my target language.

Using these explicit learning tools (and following the Weak Interface model) helps me to quickly gain a solid foundation in my target languages, at a rate that is much faster than I could accomplish using comprehensible input alone.

After that foundation is built, and I've reached a fluent level in the language, I can then transition to a learning style more dependent on implicit learning, as Krashen suggests.

And this is what I suggest for you, as well. Get lots of comprehensible input, but don't try to make implicit learning all you do. There is still a real, functional use for materials that encourage explicit learning, like translations, grammar explanations, and even subtitles. For example, Lingopie and LingQ (affiliates) are amazing tools that you can use to learn your target language by watching your favorite shows.

Thanks to this, you should instead combine both implicit and explicit learning content so that you can learn your target language both quickly and effectively.


Alright, that's all I have for you today. But before we go, let's review what we've learned.

Stephen Krashen, one of the foremost linguists in the field of second language acquisition, believes that languages can ONLY be learned through use of comprehensible input, or input that is just above your current skill level.

While I personally do acknowledge that comprehensible input is the best kind of input for language learning, I don't believe that it's all we need—there are still a few puzzle pieces we need in order to learn languages fast, and to a high level.

Differently from Krashen, I believe that certain "explicit" learning tools (like translations, grammar explanations, and even feedback given in your native language) can help you learn a new language while saving lots of time that you would otherwise spend waiting for your brain to implicitly "absorb" the language!

As such, I believe it is best to learn new languages in a way that combines both Krashen's implicit learning methods and some of the more typical methods for explicit learning that we're already familiar with. This is because explicit learning FACILITATES the acquisition of implicit knowledge, which is essential to learning a language well.

How do you learn best? Implicitly, explicitly, or somewhere in between? Feel free to share it with me in a comment down below!

Written by Luca Lampariello

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  • Krashen also mentions the (mental) Monitor. The Monitor is something like an inbuilt grammar checker. I think the Monitor can work from knowledge acquired both explicitly and implicitly. The question is if the Monitor facilitates the learning or becomes an obstacle.
    A practical example of a useful Monitor would be a native speaker who at once can tell if something is correct in his or her native language.
    For a second language speaker (a learner), his or her Monitor will mostly use grammar rules and memorized collocations and phrases to check the language.
    Please forgive if my explanation is not completely correct.
    What is your take on the Monitor?

  • Hi Luca. Just to add a possible counterexample to Krashen’s (unadapted) theory – fossilised language errors often persist despite a learner having assimilated and lived in a country for a number of years or decades. Perhaps in terms of speech, pragmatism overules interest in accuracy if the learner can fluently get their message across. All the best, Tom

  • What Krashen says is accurate in many ways, (for example, the importance of comprehensible input and a low anxiety environment). But Krashen clearly knows virtually nothing of neuroscience—and to give him credit, at the time Krashen made the video, neuroscience had nothing like the insight it provides today.

    For example, Krashen implies that understanding is all you need to learn a language. That’s perhaps appropriate to say for kids, but not adults (note that all his examples involved children).

    Neuroscientific evidence is increasingly revealing that adults have weaker procedural systems, so drill for adults can help facilitate the development of the procedural, intuitive sets of neural links that are so important for adults as well as children when it comes to language learning. This fading procedural learning system appears to be related to why infants and toddlers are able to pick up languages with ease—where they cannot do it so easily at age 10, and not nearly as easily at age 20. A fascinating recent book related to this topic is The Cognitive Unconscious: The First Half Century, edited by Arthur Reber and Rhianon Allen.

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