Modal Verbs

Modals will often be seen in sentences that are predicting a future possibility, describing an ability, giving advice, making requests, or asking for permission. The nine most common modals are can, could, shall, should, will, would, may, might, and must. 

Perfect modals – such as would have, could have, and should have – help us travel back in time. While the simple past is used to talk about what happened in the past, perfect modals go further and allow us to express those thoughts/feelings about the past.