Bare 不 just denies. 絕不 cuts the possibility off at the limit: not one exception survives, where plain 不 says only that it is not so.
並不
並不 denies a presumption the listener holds (contrary to what you'd think, not so). 絕不 makes no answer to an assumption; it drives the denial to its absolute edge.
從來不
從來不 sweeps the whole past habit (never once, up to now). 絕不 sweeps possibility itself, often forward: under no circumstances, exception included.
weakened to a plain denial: 他絕不來 meaning merely 'he isn't coming' → 他不來 (絕 is for a categorical shut-out, not an ordinary no)
wrong negator for a non-event: 他絕不去過 → 他絕沒去過 (the going never happened → 沒)
絕 detached from the negator: 絕他不答應 → 他絕不答應
English packs the force into 'absolutely', 'never', or stress ('I will NOT'); learners reach for these without 絕, and the denial drops to an ordinary 不 with the limit gone.