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This is a subtle point. In effect, both teachers and students are deceiving themselves. They believe that it is the subject matter itself, the study of grammar, that is responsible for the students progress, but in reality their progress is coming from the medium and not the message. Any subject matter that held their interest would do just as well.

Universal grammar

From the field of linguistics, the most influential theory by far has been Chomsky's theory of Universal Grammar(UG). The core of this theory lies on the existence of an innate universal grammar, grounded on the poverty of the stimulus. The UG model of principles, basic properties which all languages share, and parameters, properties which can vary between languages, has been the basis for much second-language research.
From a UG perspective, learning the grammar of a second language is simply a matter of setting the correct parameters. Take the pro-drop parameter, which dictates whether or not sentences must have a subject in order to be grammatically correct. This parameter can have two values: positive, in which case sentences do not necessarily need a subject, and negative, in which case subjects must be present. In German the sentence "Er spricht" (he speaks) is grammatical, but the sentence "Spricht" (speaks) is ungrammatical. In Italian, however, the sentence "Parla" (speaks) is perfectly normal and grammatically correct. A German speaker learning Italian would only need to deduce that subjects are optional from the language he hears, and then set his pro-drop parameter for Italian accordingly. Once he has set all the parameters in the language correctly, then from a UG perspective he can be said to have learned Italian, i.e. he will always produce perfectly correct Italian sentences.
Universal Grammar also provides a succinct explanation for much of the phenomenon of language transfer. Spanish learners of English who make the mistake "Is raining" instead of "It is raining" have not yet set their pro-drop parameters correctly and are still using the same setting as in Spanish.
The main shortcoming of Universal Grammar in describing second-language acquisition is that it does not deal at all with the psychological processes involved with learning a language. UG scholarship is only concerned with whether parameters are set or not, not with how they are set. Schachter^[5] (1988) is a useful critique of research testing the role of Universal Grammar in second language acquisition.