Stephen Krashen, a linguist, propounded the "Theory of Second Language Acquisition", in which he proposed the five main hypotheses which include Input Hypothesis, Acquisition Learning Hypothesis, Natural Order Hypothesis, Monitor Hypothesis, and Affective Filter Hypothesis.Let's Understand in Brief:Acquisition Learning Hypothesis: According to Krashen, there are two ways for second language learners to develop knowledge of a second language: 'acquisition' and 'learning'.In his view, we acquire as we are exposed to samples of the second language which we understand.Monitor Hypothesis: Krashen argues that the acquired system acts to initiate the speaker's utterances and is responsible for fluency and intuitive judgments about correctness. The learned system, on the other hand, acts only as an editor or 'monitor', making minor changes and polishing what the acquired system has produced.The Natural Order Hypothesis:It suggests that children acquire their first language in a predictable order and children's age, deliberate teaching, etc. don't influence the natural order of acquisition. In this approach, students are given plenty of comprehensible input by the teacher, which facilitates the acquisition of language.Input hypothesis:It refers to the relationship between what the learner is exposed to a language and language acquisition. It helps learners in language learning by making them able to communicate efficiently. It emphasizes putting comprehensible input slightly above the learner's current level.Affective Filter Hypothesis: The 'affective filter' is an imaginary barrier, which prevents learners from acquiring language from the available input. 'Affect' refers to such things as motives, needs, attitudes, and emotional states. A learner who is tensed, angry, anxious, or bored may filter out' input, making it unavailable for acquisition. Hence, it could be concluded that Krashen's Natural order Hypothesis describes that parts of languages are acquired in a predictable order.