clitic (plural clitics)
(linguistics) A morpheme that functions like a word, but never appears as an independent word, instead being always attached to a following or preceding word (or, in some cases, within a surrounding word).
e.g. It’s, should’ve, shouldn’t’ve.
There are 2 reasons [for why/when clitics can be applied]
Syntatic Gap — A clitic implies another word shoud follow
e.g. “Who do you think you’re _____?”
Stress patterns — These forms are generaly unsrtesse or week. They’re pronounced more quietly and less distinct.
e.g. you cannot stress a clitic and sounds weird if you do.