1. Language universals are those patterns or phenomena which occur in all languages. In contrast, there are patterns which vary across languages called parameters. This is the patterns which cause difficulty in language learning. Interlanguge is formed due to existence of parameters in languages, partially.
2. Markedness theory states that within and across languages, some structures are unmarked, i. e. simple, while others are marked, i. e. complex, exceptional. According to Markedness Differntial Hypothesis, if TL contains structures that are marked, these will be difficult to learn. Such difficulties are observable in a learner’s interlanguage.
3. Language transfer is defined as the effect of one language on the learning of another. Through his interlanguge, a leaner borrows pattens from his mother tongue. These patterns which agree with TL only in form constitute part of a person’s interlanguage.
4. Input is the language a learner hears or receives and from which he can learn. According to input hypothesis, exposure to comprehensible input (i+1) is the necessary cause of second language acquisition. Talking about input, we should refer to the concept ‘intake’. Intake is the input that actually ‘goes in’ , that is, is internalized by the learner. The more intake a learner has, the faster his interlanguage develops.