Why Comprehensible Input Works (and Flashcards Aren’t the Whole Story)

Let’s be clear: there’s nothing wrong with using flashcards or spaced repetition systems (SRS). In fact, research consistently shows that spaced repetition is a powerful way to help encode vocabulary into long-term memory. Tools like Anki are excellent for strengthening recall and building a foundation of core vocabulary.

But here’s the thing: flashcards are not the whole story.

Memorizing isolated words doesn’t teach you how those words function in real sentences, how they shift in tone or meaning depending on the situation, or how native speakers naturally use them. That’s where comprehensible input comes in - the idea that language is best acquired when we’re exposed to messages that are slightly above our current level - but still understandable. You don’t need to grasp every single word; you just need enough context for meaning to emerge.

Here, Stephen Krashen explains:

If you didn't watch the video, here’s the main idea Stephen Krashen presents:

He explains that we acquire language in only one way:

"When we understand it."

This is what’s commonly called comprehensible input.

Krashen emphasizes that:

"We don't acquire language when we speak,
we don't acquire language when we study it,
we don't acquire language when we memorize lists of verbs etc. etc."

All of those things - grammar, vocabulary - are not the source of acquisition.

Rather, he says:

"All those things like grammar, vocabulary, are the result of getting comprehensible input.
This is how we acquire language."

Krashen distinguishes between two different processes:
You can either acquire language or learn language - and, as he says:

"They're very, very different."

He describes acquisition as a subconscious process:

"While it's happening you don't know it's happening.
Once you're done acquiring something you don't even know that it's there."

According to Krashen:

"The brain is very, very good at acquiring language."

But learning - the conscious study of grammar rules and language structure - is different:

"The other process is called learning.
That’s knowing about language - the subject and the verb are supposed to agree, etc.
The brain is not very good at learning language."

And unfortunately:

"We have spent most of our time in classes trying to make people consciously learn -
and it’s been pretty torturous."

- Stephen Krashen

Input vs Output (Two sides of the same coin)

While Krashen makes a compelling argument for why comprehensible input is needed to acquire language, what does 'acquiring' mean exactly and is it the same thing as being fluent? Well, not quite..

While comprehensible input lays the foundation for language acquisition, it’s only one side of the coin. To truly develop fluency, we also need to produce language - to speak and write it actively. This is where output comes into play. In a previous article The Shadowing Technique, the Output Hypothesis was referred to as being a key part

Proposed by Merrill Swain, language learners don't just demonstrate what they know when they speak - they actually develop their language skills through the act of producing language.

In short, Output helps us notice gaps in our knowledge, solidify grammatical structures, and move passive vocabulary into active use.

Just as input gives us the building blocks of language, output helps us assemble them into something usable. In other words, input and output are not opposites - they’re partners. Engaging with language receptively and expressively creates a feedback loop that accelerates progress and deepens true fluency.

Forget in order to Remember!

Crucially, memory is a process of forgetting and being reminded. That’s why repeated, meaningful exposure is so important. Research shows that we need to encounter a word in multiple varied contexts - not just on a flashcard - to move it into our active vocabulary. Some studies suggest we may need 10 to 20 contextual encounters for a word to really “stick”. This is where Comprehensible Input shines through!

Memory is a process of forgetting and being reminded.

Final Thoughts

Krashen’s theory highlights a core concept, that understanding comes first. If you're learning a language, focus on input you can make sense of - stories, conversations, videos, or books just a little above your current level. That’s where the real progress happens - not with flashcards or grammar drills, but in the moments when the meaning clicks.

Motivated to acquire a language through comprehensible input and practice output at the same time? HearSay Learn offers personalized, comprehensible audio lessons in your target language to learn -screen free- while out and about.