t. Why? Robin washed the dog's expensive new collar is a perfectly good, probably unique English sentence, but washed expensive the Robin collar dog's new is utter gibberish. Why? Descriptive linguistics is not concerned with explaining why, but generative linguistics is. Generative linguists have set for themselves the daunting task of accounting for the infinite, yet constrained, possibilities of language using a finite set of rules.
The names of the two schools are themselves quite telling. The point of descriptive linguistics is simply to describe a linguistic fact or set of facts. Picking up the phonotactic example above, given the English words please, proud, and pure, descriptive linguists would state that, in English, word-initial /p/ may be followed by either /r/, /l/, or /j/ and be satisfied. Generative linguists would find this rule
...