The imperative mood indicates a command,
Put that on the press.
The subject of the imperative mood is only expressed when it is emphatic,
Go thou and do likewise.
Older grammarians speak of a fourth mood called potential. The present tendency among grammarians is to treat these forms separately. They are verb phrases which express ability, possibility, obligation, or necessity. They are formed by the use of the auxiliary verbs may, can, must, might, could, would, and should, with the infinitive without to.
May is used (a) to show that the subject is permitted to do something, "You may go out," or (b) to indicate possibility or doubtful intention, "I may not go to work tomorrow."
Can is used to show that the subject is able to do something, "I can feed a press." These two forms are often confused, with results which would be ridiculous if they were not too common to attract attention. The confusion perhaps arises from the fact that the ability to do a thing often appears to depend on permission to do it. "May I see a proof?" means "Have I permission, or will you allow me, to see a proof?" and is the proper way to put the question. The common question, "Can I see a proof?" is absurd. Of course you can, if you have normal eyesight.
Must shows necessity or obligation.
You must obey the rules of the office.
Ought which is sometimes confounded with must in phrases of this sort expresses moral obligation as distinguished from necessity.