To add a tone of politeness and friendliness in a host-guest situation, the Imperfective aspect of Imperatives is used instead of the Perfective, e.g. cliches like Come in! Sit down! Take off your things! Do drop by sometime! Do come! They sound more polite because you are not telling the other person to complete the action, to get something done: you are merely telling him to start doing it. A Perfective verb suggests that you want it completed. Thus your host is likely to say Снимђйте пальтћ, садќтесь "Take off your coat and sit somewhere" while a doctor is more likely to say Снимќте рубђшку и сїдьте на стћл "Take off your shirt and sit on the table," because he is giving you specific directions that he wants you to follow to the end.
The meaning of some verbs may preclude their ever being used in a "polite" imperative form, e.g. Leave me alone! Отстђнь от менї! (Perfective only).
(For an even stronger command, infinitives are used; a dog trainer would say Сидљть!)
Impf. Садќтесь! Sit down! Have a seat!
Pf. Сїдьте! Sit down! Sit!
Impf. Входќте! Come in! Do come in!
Pf. Войдќте! Come in!
Impf. Молчќте! Be quiet!
Pf. Замолчќ! Shut up!