The verbs can, could, may, might, will, would, shall, should, must and ought are called modal auxiliary verbs. Modal auxiliary verbs are used before infinitives to add certain kinds of meaning associated with certainty or with obligation.
Modal auxiliary verbs usually exhibit the following properties.
1. Modal verbs have no -s in the third person singular.
- She can knit. (NOT She cans knit.)
- She may come. (NOT She mays come.)
2. Modal verbs form their questions and negatives without do.
- Can she knit? (NOT Does she can knit?)
- She won’t be coming. (NOT She doesn’t will be coming.)
3. Modal auxiliary verbs are followed by infinitives without to.
- She should go. (NOT She should to go.)
- You must wait. (NOT You must to wait.)
Note that ought is an exception to this rule. It is followed by an infinitive with to.
- He ought to understand. (NOT He ought understand.)
4. Modal auxiliary verbs do not have infinitives or participles.
For example, we cannot say to may, maying or mayed. When necessary other expressions are used.
- I would like to be able to paint. (NOT I would like to can paint.)
- People had to work hard in those days. (NOT People musted work hard in those days.)