I+ 1 theory: one of a series of hypotheses put forward by Krashen on second language acquisition: the way for human beings to acquire language is to understand information, that is, to acquire language knowledge by absorbing understandable input. Therefore, it is neither too difficult nor too easy for teachers to convey language information to students in classroom teaching. Krashen is represented by "i+ 1". I refers to the student's current language level, and i+ 1 refers to the level that the student will reach in the next stage, that is, slightly higher than his current level, so that he can understand the information of i+ 1 through context and other non-verbal means such as teaching AIDS and PPT, so as to master the language structure contained in the information.