May, might, can and could can refer to both the present and the future. They can be used after if:
But when could is the past form of can, it can only refer to the past or indicate unreality:
Must has a special meaning in questions - it can express annoyance:
In if-clauses it's used in a similar way:
In other uses, had to can refer to the past or indicate unreality:
When modal verbs are used in polite requests, they can be used with if in the same way:
This includes will and would:
In such situations an if-clause doesn't need a main clause.