Hypothesis 1 Acquisition versus Learning
According to Krashen, there are two entirely separate processes at work in building up knowledge of a second language: acquisition, which involves the implicit, unconscious creation of a grammar in the same way as a child acquires its first language; and learning which involves building up language through explicit, conscious knowledge in such activities as rule-learning, drill and error correction.

Hypothesis 2 Learning-as-Monitor
Krashen claims that learned knowledge can only function as a Monitor to help correct production in the L2. The real knowledge that enables us to perform fluently and spontaneously comes from acquisition:

Acquisition 'initiates' our utterances in a second language and is responsible for our fluency. […] Learning comes into play only to make changes in the form of our utterance, after it has been 'produced' by the acquired system. (Krashen, 1982: 15)

This, Krashen argues, is because the brain can only access learned knowledge when there is time to think and when the learner's focus is on form. Since these conditions don't usually hold in normal language use, learned knowledge is virtually impossible to use. (By learned knowledge Krashen seems to be referring to declarative knowledge - see section 2.2.1.5.)

Hypothesis 3 The Natural Order
The second language grammatical system is acquired in a predictable 'natural' order, which cannot be influenced by formal teaching and is not determined by the learner's first language. This hypothesis draws its support from the research we have already reviewed in section 2.1.4 which shows predictable stages of development across L2 learners from a variety of backgrounds in such areas as negation, question formation and grammatical morphemes of English.

Hypothesis 4 Input
This claims that all that is required for second language acquisition is input which is comprehensible to the acquirer. This hypothesis relies strongly on the arguments from studies of child-directed speech in first language acquisition (see section 2.2.3.3) which showed that adults instinctively adjust the complexity of their speech to different levels of acquisition in children, as well as studies which show native speakers simplifying their speech to non-native speakers (more on this in section 2.3.3.2). Thus, Krashen argues, input to learners doesn't have to be specially structured, as in the behaviourist view; we simplify our language quite naturally in the process of trying to make target language input comprehensible to a learner, and this provides learners with what they need for acquisition. He further argues that getting learners to produce language should not be the emphasis in language teaching. As he says:

Speaking fluency emerges over time, on its own. The best way, and perhaps the only way, to teach speaking is to provide comprehensible input. Production ability emerges. It is not taught directly. (Krashen, 1982: 22)

Hypothesis 5 The Affective Filter
If language acquisition fails to take place despite exposure to comprehensible input, then Krashen claims that this must be due to affective factors (motivation, self-confidence, anxiety) to which second language learners are more susceptible than children acquiring their first language. These factors act like a filter, stopping comprehensible input from triggering acquisition:

Those whose attitudes are not optimal for SLA will not only tend to seek less input, but they will also have a high Affective Filter - even if they understand the message, the input will not reach that part of the brain responsible for language acquisition, or the language acquisition device. Those with attitudes more conducive to SLA will not only seek and obtain more input, they will also have a lower or weaker filter. They will be more open to the input, and it will strike 'deeper'. (Krashen, 1982: 31)

Reflective task 16