Processability theory is a theory and a model of second language acquisition developed by Manfred Pienemann that touches the linguistic structures that surface in the learning of a second language. The theory has been used as a framework by several scientists from Europe and Australia.Processability theory is part of the cognitive approach to second language acquisition that attempts to increase understanding of the ways L2 learners restructure their interlanguage knowledge systems to be in greater conformity to L2 structures. Processability theory states that learners restructure their L2 knowledge systems in an order of which they are capable at their stage of development. For instance, In order to acquire the correct morphological and syntactic forms for English questions, learners must transform declarative English sentences. They do so by a series of stages, consistent across learners. Clahsen proposed that certain processing principles determine this order of restructuring. Specifically, he stated that learners first, maintain declarative word order while changing other aspects of the utterances, second, move words to the beginning and end of sentences, and third, move elements within main clauses before subordinate clauses.