Modal Verbs: Properties

Modal verbs can be distinguished according to the following properties:

1) the verb that follows does not take the infinitive form:

He should be arriving anytime now.

2) it does not appear in the same sentence with another modal verb:

  X I don’t can play the guitar. X

3) they show a mood (such as an obligation):

You must do your homework now.
You have to do your homework now.

Modal verbs can take a past form (“it must have been him”) but not a future form (X”It will must be him”X).