There are two schools of thought when teaching a language. We'll use English as an example here.
There are two types of people when teaching English:
1. the English teacher
2. the linguist
Essentially, English teachers are full of shit. Language is not static. That is it evolves and changes with time, this includes words, syntax and morphology.
The English teacher is a prescriptivist (or Linguistic Prescrption)...they believe that there is a right and wrong way to speak or a 'proper' way to speak.
The linguist is a descriptivist, (descriptive linguistics) and is interested in analyzing (see semantics for how this is done and syntax) and describing how language is spoken in a given community at a given time (not a short period, like weeks or a few years).
In other words...English teachers are concerned with how language should be (keeping it static and enforcing social and politically correct norms (i.e. see who/whom argument) while the others, the linguists, declares how language is.