In the if-clauseA group of related words containing a subject and a verb. of the first conditional we can use should (to express that the condition is less likely), can (permission or ability), will or would (willingness, insistence and refusal), must (obligation) or may (permission).

Instead of will here we can use may/might (certainty/uncertainty), may (permission), can (permission or ability) or must/should (obligation).

Must and have to are more or less the same, they are often interchangeable. However, taking a closer look, we will find a difference.

To express past certainty, the same auxiliariesA helping verb used with main verbs to express mood, voice and tense. E.g.: I can't stand when I am disturbed. ("can", "am") are used as in the case of present and future certainty. However, the infinitives that follow are perfectThe aspect expressing completion. E.g.: He’s finished university. infinitives.

Here we discuss structureThe way in which the parts of a sentence, clause or expression are arranged. E.g.: make somebody + infinitive in "The teacher made me rewrite the composition."s that show how certain the speaker is whether something happens or will happen. Between the two extremes (the first and the last sentence below) the order of the structures (might, may, could etc.) expresses a growing amount of certainty.