Quote Originally Posted by justinwyllie View Post
Малчай?
The imperative form of the verb молчать ‘be silent’ is молчи́ (sg.) / молчи́те (pl.).
You can use this form, but more commonly, it is used with a prefix:
замолчи́(те) — literally: ‘start being silent’, or
помолчи́(те) — literally: ‘be silent for a while’.
You can also use the infinitive form without prefixes: молча́ть!

All these phrases are quite direct requests—neither polite nor too rude.

If you are addressing several people, you should use plural forms with -те.
If you are addressing a single person, the choice between singular and plural forms is not trivial. It depends on social relations between interlocutors and personal preferences. Usually teachers in schools address their (pre)teenage students with the pronoun ты and singular verb forms, while professors in collages and universities address their adult students with the pronoun вы and plural forms (singular forms can be used to increase rudeness). This can vary with individual teachers/schools however.

You can also see this topic for phrases with similar meaning and different degree of rudeness.