second-language acquisition is concerned with the internal representation of a language in the mind of the learner, and how those representations change over time. It is not yet possible to inspect these representations directly with brain scans or similar techniques, so SLA researchers are forced to make inferences about these rules from learners' speech or writing. Interlanguage Originally, attempts to describe learner language were based on comparing different languages and on analyzing learners' errors. However, these approaches were unable to predict all the errors that learners made when in the process of learning a second language. For example, Serbo-Croat speakers learning English may say "What does Pat doing now?", although this is not a valid sentence in either language. Additionally, Yip found that ergative verbs in English are regularly mis-passivized by L2 learners of English whose first language is Mandarin. For instance, even advanced learners may form utterances such as "what was happened?" despite the fact that this construction has no obvious source in either L1 or L2. This could be because L2 speakers interpret ergatives as transitive, as these are the only types of verbs that allow passivization in English. To explain this kind of systematic error, the idea of the interlanguage was developed. An interlanguage is an emerging language system in the mind of a second-language learner. A learner's interlanguage is not a deficient version of the language being learned filled with random errors, nor is it a language purely based on errors introduced from the learner's first language. Rather, it is a language in its own right, with its own systematic rules. It is possible to view most aspects of language from an interlanguage perspective, including grammar, phonology, lexicon, and pragmatics. Three different processes influence the creation of interlanguages: Language transfer. Learners fall back on their mother tongue to help create their language system. Transfer can be positive, i.e. promote learning, or negative, i.e. lead to mistakes. In the latter case, linguists also use the term interference error. Overgeneralization. Learners use rules from the second language in roughly the same way that children overgeneralize in their first language. For example, a learner may say "I home", overgeneralizing the English rule of adding -ed to create past tense verb forms. English children also produce forms like , , and . German children equally overextend regular past tense forms to irregular forms. Simplification. Learners use a highly simplified form