가

Slipped into a negative for emphasis

All eight lines here are the same construction: the negative 지 않다 with 가 wedged into the middle. It adds emphasis and a note of complaint — it just won't tear cleanly, I can't get used to you calling these biscuits, my heart doesn't feel at ease, they weren't very cooperative. Without the 가 the sentences mean the same thing but flatter; with it, the speaker is pushing back against how things are. This is the subject marker being borrowed for emphasis, not marking a subject.

Register: casual, spoken