Stephen Krashen is one of the most influential linguists in the field of second language acquisition (SLA). In the late 1970s and 1980s, he proposed a set of ideas—often referred to collectively as Krashen’s Theory of Second Language Acquisition—that focus on how people naturally acquire languages. His theory is made up of five main hypotheses, all of which emphasize the importance of meaningful communication rather than explicit grammar instruction.
The first is the Acquisition–Learning Hypothesis. Krashen argues that there are two distinct ways of developing ability in a second language. Acquisition is a subconscious process, similar to how children learn their first language, through exposure and interaction. Learning, on the other hand, is a conscious process that involves knowing rules and grammar. According to Krashen, true fluency comes from acquisition, not learning, and learned knowledge can only play a limited role.
The second is the Monitor Hypothesis. This hypothesis explains how learned knowledge is used. Krashen claims that consciously learned grammar acts as a “monitor” that can edit or correct language output, but only under specific conditions: the speaker must have enough time, focus on form, and know the rule. Because these conditions are rarely met in real communication, Krashen believes the monitor has a small and often overused role, especially in classroom settings.
The third and most well-known idea is the Input Hypothesis. Krashen proposes that language acquisition occurs when learners are exposed to comprehensible input—language that is slightly above their current level of competence. He refers to this as i + 1, where i is what the learner already knows, and +1 represents new, understandable language features. According to this view, speaking ability develops naturally as a result of understanding input, rather than from forced practice.
The fourth is the Affective Filter Hypothesis. This hypothesis highlights the role of emotions in language learning. Factors such as anxiety, low motivation, and lack of confidence can raise a learner’s “affective filter,” blocking input from being acquired. When learners feel relaxed, motivated, and confident, the affective filter is low, allowing language acquisition to happen more effectively.
The final component is the Natural Order Hypothesis. Krashen suggests that grammatical structures are acquired in a predictable sequence, regardless of the learner’s first language or instruction. This means that teaching grammar in a specific order does not necessarily change the natural acquisition sequence.
Overall, Krashen’s theory shifted attention away from grammar-heavy instruction toward meaningful communication, extensive listening and reading, and supportive learning environments. While some aspects of his theory have been debated, his ideas continue to strongly influence language teaching practices around the world.