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).