All modal verbs are auxiliary verbs, which means they can only be used with a main verb. Modal verbs cannot be a main verb. They are used to express ideas such as possibility, prediction, speculation, deduction and necessity.
Shades of Modality [infographic]: A modal verb is a type of auxiliary verb that expresses possibility, necessity or obligation. Because of their commanding tone, modals must be used carefully, and some writers attempt to avoid them entirely.
The correct usage of CAN and COULD in American Casual, Spoken English using "Modal Verbs", general rules to go by (never hard and fast or forever, always, aka: always exceptions but here's a really good start)