Modal verbs are helping verbs and they help main verbs to express or to describe a host of situations. Modal verbs are also called auxiliary verbs and they are always used with main verbs as they cannot stand on their own. The use of modal verbs is governed principally by the 9 pure modals - CAN, COULD, MAY, MIGHT, MUST, SHALL, SHOULD, WILL and WOULD. Many situations can be explained or described by the use of modal verbs and it is possible that a modal verb can explain different situations and also a situation can be expressed by different modal verbs.
Specific rules govern the use of modal verbs:
Modal verbs are followed by the base form of a verb – "I MUST COMPLETE my assignment well in time."