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).

To express permission and prohibition, the following 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 are used.

To express offers, usually the 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") of permission, possibility, suggestion and willingness are used.

The auxiliaryA helping verb used with main verbs to express mood, voice and tense. E.g.: I can't stand when I am disturbed. ("can", "am") can is used here to express capability.

Winters in Minnesota can be really cold.

For annoying habits, typical of a person we can use would. This usage sometimes implies criticism and has an emphaticSpecial importance or attention given to something. E.g.: All I wanna do is have some fun. effect:

To show disapproval, the 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") can, could, might and should/shouldn't are used.

You can be really annoying, you know! 
You shouldn't be sitting here just doing nothing!
You might be more polite!
George could really help you!

For future ability, both the auxiliaryA helping verb used with main verbs to express mood, voice and tense. E.g.: I can't stand when I am disturbed. ("can", "am") can and the future forms of be able to are used.

If you ask him, he can probably help you. 
If you ask him, he will probably be able to help you.

To express that someone has the power or skill to do something, the auxiliaryA helping verb used with main verbs to express mood, voice and tense. E.g.: I can't stand when I am disturbed. ("can", "am") can and the semi-auxiliary be able to is used.

I can swim.
We can't enter this street because it's blocked.
I am not able to help you at the moment.

Normally, the 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") of permission, possibility and willingness are used to express requests.