Stephen Krashen is a famous language researcher. He said that we learn a language by understanding messages that are a little difficult for us. He called this 'i + 1'. Reading easy books in English really helps us learn many words.

But some children in Canada studied in French for years. They understood French very well, but their speaking and writing still had grammar mistakes. So input is important, but it may not be enough.

In the 1970s and 1980s, the American linguist Stephen Krashen proposed the input hypothesis. He argued that we acquire a language by understanding messages slightly above our current level, which he called 'i + 1', and that speaking ability appears naturally once we have enough input. He also suggested that anxiety can block learning, an idea called the affective filter. There is good evidence that reading a lot helps learners build vocabulary.

However, critics pointed out that 'i + 1' is hard to measure. Also, in Canadian immersion programs, children who studied in French for years understood it very well but still made grammar errors when speaking and writing.

Stephen Krashen's input hypothesis is one of the most influential ideas in language education, and one of the most contested. Krashen claimed that learners acquire a language by understanding messages slightly beyond their current level, which he called 'i + 1'. Speaking, he argued, emerges on its own once enough comprehensible input has been received. He also proposed an affective filter, through which anxiety blocks input from being acquired.

There is solid evidence on his side. People who read more tend to have larger vocabularies and better spelling, and extensive reading of easy books improves vocabulary and reading speed. Critics raised two main objections. Gregg argued that because 'i + 1' was never clearly defined, the theory could explain any result and was therefore difficult to falsify.

White noted that input alone rarely tells learners what is impossible in English, such as word orders that are natural in their first language but not in English. Canadian immersion research by Swain and colleagues pointed the same way. After years of French-medium schooling, students understood French extremely well, but their grammatical accuracy in speaking and writing lagged behind that of native speakers. Most researchers now treat input as necessary but not sufficient.