Forming Simple and Compound Verb Tenses

A compound tense places an action in time by stacking one or more helper verbs in front of a main verb. The helpers do the grammatical work — marking whether something is ongoing, finished, or still being completed — while the main verb carries the meaning. Common helpers include forms of have, be, and the modals: would, could, should, might, must. Every slot in that stack has to be filled by a verb. That is the whole rule.

Build 'She has been waiting' piece by piece. Has is a helper showing present perfect. Been is a second helper, a form of be, showing the action is ongoing. Waiting is the main verb, carrying the -ing form that pairs with been. Three pieces, one job: an action that started in the past and is still happening now. Swap out any piece and the sentence either breaks or shifts meaning.

This is why 'would of,' 'could of,' and 'should of' never work. In speech, 'would have' contracts to 'would've,' and 'would've' sounds just like 'would of.' But of is a preposition — it links nouns, as in 'a cup of coffee.' It cannot sit in a verb stack, because that slot needs another verb. 'Should have called,' 'could have caught,' 'would have gone' are the only correct forms. This sits right next to subject-verb agreement in the same unit: both are about recognizing which exact word a slot demands, even when the ear is tempted by something that sounds close enough.