Definition of modal verbs

Modal verbs are special verbs which behave very differently from normal verbs. Here are some important differences:
1. Modal verbs do not take "-s" in the third person.

2. You use "not" to make modal verbs negative, even in Simple Present and Simple Past.

3. Many modal verbs cannot be used in the past tenses or the future tenses.

Common Modal Verbs

Can Could May Might Must

Ought to Shall Should Will Would