Substitutes for modal verbs are necessary when the corresponding modal verb lacks a form for a specific verb tense—for example, future or past. In such cases, we use their substitutes.
modal verb / substitute
CAN / BE ABLE TO
MAY / BE ALLOWED TO
MUST / HAVE TO, BE SUPPOSED TO
Examples:
present tense past tense
He can do it. He could do it.
(He can do it.); He was able to do it.
(He was able to do it.)
future tense
He will be able to do it. (He will be able to do it.)
Here is an example with must:
Present tense / past tense
He must do it. He had to do it.
(He must do it.) (He had to do it.)
future tense
He will have to do it. (He will have to do it.)