Comprehensible Input: Krashen's i+1 Hypothesis Explained

Stephen Krashen formulated the idea in the late 1970s and developed it in two books: Principles and Practice in Second Language Acquisition (1982) and The Input Hypothesis: Issues and Implications (1985). It is one of the most influential ideas in language teaching and also one of the most debated. To put it into practice you need input at your level and, as we'll see, English speaking practice with an AI coach or with people.

If your problem is that you understand but can't speak, also read understand English but can't speak it? And if you want to strengthen grammar without memorizing lists, see English grammar practice online by level.

What is Krashen's input hypothesis?

It's the idea that second language acquisition happens when we understand messages. In Krashen's words (1982, pp. 20–21), a necessary, but not sufficient, condition to move from the current level (i) to the next one (i+1) is that the acquirer understands input that contains i+1, where "understand" means focusing on the meaning and not the form of the message.

In the 1982 book, Krashen breaks the hypothesis into four parts:

1. It relates to acquisition, not learning. For Krashen, "acquiring" is a subconscious process, like a child with their first language. "Learning" is conscious knowledge of the rules.
2. We acquire by understanding language slightly beyond our level (i+1). We manage it thanks to context, our knowledge of the world and extra-linguistic information.
3. If communication is successful and there's enough input, i+1 is provided automatically. That's why Krashen advises against a "structure of the day" syllabus.
4. Speaking ability "emerges". It isn't taught directly: it arrives over time, when the learner feels ready.

What does i+1 mean?

The "i" is your current competence and the "+1" is the next step in your development. An i+1 text is one you understand almost completely, but it includes something new you can work out from context. If everything is new (i+10), you understand nothing and acquire nothing. If nothing is new (i+0), you understand everything but don't move forward either.

Krashen never gave an exact percentage. A practical reference comes from research on vocabulary and reading: Schmitt, Jiang and Grabe (2011) studied 661 readers from 8 countries and found a relatively linear relationship between the percentage of words known and comprehension. They concluded that knowing around 98% of the words is a reasonable target for academic texts. In practice: if you don't know more than 2 or 3 words in every 100 on a page, you're probably above i+1.

Krashen's five hypotheses

The input hypothesis is one of five that make up his theory (the "Monitor Model"), as listed in chapter II of the 1982 book:

HelloVozi table based on Krashen (1982) and the sources cited in each row, accessed Sep 29, 2026.

Comprehensible input examples by level

The most useful part of the hypothesis is practical: find lots of input you almost fully understand. Here are examples for each CEFR level. If you don't know your level, check our guide to CEFR levels from A1 to C2.

Examples suggested by HelloVozi. Levels follow the CEFR global scale.

How to get comprehensible input in 5 steps

1. Choose by interest, not just by level. If the topic interests you, you understand more from context and listen for longer.
2. Try the 98% test. Read a page: if there are more than 2 or 3 unknown words per 100, step down a level.
3. Go for quantity. Krashen insists there must be "enough" input. Twenty minutes a day beats two hours on a Sunday.
4. Reread and relisten. The second time you understand more and the "+1" becomes visible.
5. Use it when you speak. Pick 2 or 3 new expressions and use them the same day in a conversation. This is where Krashen's theory needs a complement, as we'll see below.

Criticisms of the input hypothesis

The hypothesis has faced major criticism since the 1980s. Knowing it helps you use the idea well:

• It's hard to test. As Lichtman and VanPatten (2021) recall, once researchers began asking what requirements theories should meet, Monitor Theory was widely criticized. The best-known critiques, such as Gregg (1984) and McLaughlin (1987), argue that concepts like "acquisition", "learning" or "i+1" are hard to define and measure. If you can't tell what i+1 is for a person, you can't prove that's what helped them progress.
• Not everything depends on meaning. Lydia White (1987) argued that part of grammatical development is internally driven and independent of context, that Krashen overestimates simplified input, and that input doesn't always show learners which incorrect forms to drop. Even so, White proposed tightening the hypothesis, not abandoning it.
• Understanding isn't enough to speak. Merrill Swain (1985) proposed the output hypothesis: producing the language is also necessary. Swain and Lapkin (1995) showed that when learners speak or write they notice what they don't know and adjust their language.
• Explicit teaching does help. Norris and Ortega's (2000) meta-analysis of 49 study samples found that focused instruction leads to large gains and that explicit instruction is more effective than implicit instruction. That contradicts the idea that teaching rules does little.

So was Krashen wrong? In a 2021 essay, Karen Lichtman and Bill VanPatten revisited his ideas 40 years later. Their conclusion is that three of them are still very much alive under new names: the distinction between implicit and explicit knowledge, ordered development, and the central role of meaningful input in every current theory of acquisition.

What it means if you're learning English

• Input is the foundation. Listening to and reading lots of material you understand is essential. No serious theory disputes it.
• But don't wait for speech to "emerge" on its own. If you want to speak, practice speaking. Combining input with conversation and feedback is what later research supports.
• Explicit grammar isn't the enemy. Understanding a rule can help you notice it in the input. What doesn't work is studying rules without using them.
• Lower the pressure. The affective filter has support: practicing without fear of mistakes helps.

If you want to review a specific structure, English grammar by level covers each topic with explanations, examples and exercises, from the present simple to the passive voice.

The affective filter has a lot to do with nerves when speaking: how to overcome your fear of speaking English covers what research on language anxiety says and what to do.

For grammar review: see the English verb tenses chart and the irregular verbs list.

Next step: if you don't know your level, take the English level test (about 15 minutes, with instant results). To keep working on this topic, see how to practice English speaking with AI in Vozi Coach.