The Science of Speaking 01 / 12 Input · Stephen Krashen 
The Professor Who Said Grammar Class Won't Make You Fluent
In the early 1970s, researchers studying learners of English found something odd.
Children and adults picked up grammar endings in roughly the same order — whatever their native language, and whether or not they had ever taken a class. The -ing ending came early. The -s in "she walks" came late, even though it is one of the first rules every textbook teaches.
One of the people behind those studies was Stephen Krashen, a linguist at the University of Southern California. If teaching a rule early didn't make people use it early, he asked, what was actually happening in their heads? In 1982 he published his answer in Principles and Practice in Second Language Acquisition — a book teachers still argue about today.
Two ways to know a language
Krashen drew a line between learning and acquisition. Learning is conscious: rules, tables, tests. Acquisition is unconscious, and it happens while you are busy understanding something else — the way children pick up their first language.
In his view, only acquisition builds the ability to speak spontaneously. The rules you learned in class work mostly as a "monitor": an inner editor that checks your sentences when you have time to think. Most learners know the result well.
Correct, slow, and silent once the conversation speeds up.
Slightly above your level
So how does acquisition happen? Through comprehensible input, Krashen said: language you understand, pitched a little beyond your current level. He called it i+1. If it is too easy, nothing new comes in. If it is too hard, it is just noise.
The filter
Krashen added one more idea that nervous learners recognise immediately: the affective filter. Anxiety, low confidence and fear of mistakes act like a barrier, so less of what you hear gets through. Two people can sit in the same lesson and take away very different amounts.
What to take from it
- Spend time every week on content you actually want to understand: a series, a podcast, a conversation. Interest is what keeps you in the input.
- Choose material you can follow almost completely. If you understand everything, go harder. If you are lost, go easier.
- Look for places where mistakes are normal. A relaxed learner takes in more — that is the affective filter working for you instead of against you.
This is how our Conversation Labs work: small groups, real topics, and a moderator who keeps the conversation at a level you can follow — plus one step.
Stop reading about speaking.
Small groups, real conversations, a moderator who corrects you. Spanish, French, English or Ukrainian — online or in Barcelona.
Try a Conversation Lab →Sources
- Bailey, N., Madden, C., & Krashen, S. (1974). Is there a "natural sequence" in adult second language learning? Language Learning, 24(2), 235–243.
- Dulay, H., & Burt, M. (1974). Natural sequences in child second language acquisition. Language Learning, 24(1), 37–53.
- Hu, M., & Nation, P. (2000). Unknown vocabulary density and reading comprehension. Reading in a Foreign Language, 13(1), 403–430. Open access
- Kremmel, B., Indrarathne, B., Kormos, J., & Suzuki, S. (2023). Unknown vocabulary density and reading comprehension: Replicating Hu and Nation (2000). Language Learning, 73, 1127–1163. doi.org/10.1111/lang.12622
- Krashen, S. (1982). Principles and Practice in Second Language Acquisition. Oxford: Pergamon. Free PDF from the author
- McLaughlin, B. (1987). Theories of Second-Language Learning. London: Edward Arnold.