“Their vocabulary caps their thinking”

Students think with the words they have, and most understand far more than they can say. In the moment of answering, the everyday word arrives first, and 'stuff' and 'makes it worse' do the job well enough. You help this along without meaning to: you can hear the understanding through the rough phrasing, so you supply the precise term yourself, and the sharpest sentence in the exchange is usually yours. Nobody is being lazy. Reaching for a half-known word while holding an idea is hard, and the familiar word wins.

It holds because the gist keeps the lesson moving. You get the idea, they feel understood, and nobody risks a word that might sound wrong in front of the class. Meanwhile new terms sit in glossaries, met once and never used, and a word only becomes a student's own through use. The techniques below work from three directions: some build the stock of words deliberately, said aloud and revisited until they stick; some hand students the language at the moment of speaking or writing; and some make precision the task itself, asking for the same idea again in sharper words.