Originally, attempts to describe learner language were based on comparing different languages and on analyzing learners' errors. However, these approaches weren't able 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.[18]
To explain this kind of systematic error, the idea of the interlanguage was developed.[19] 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.[20] It is possible to view most aspects of language from an interlanguage perspective, including grammar, phonology, lexicon, and pragmatics.