Use the Perfective partner to talk about the specific instance; use the Imperfective partner to talk about doing the same thing in general, e.g. as a matter of habit or principle. If you have an adverb like всегдђ 'always' or чђсто 'often' in a sentence, you have to use the Imperfective aspect of the verb.
ВІ положќли кларнљт на пћлку? (положќть Pf.) Did you put the clarinet on the shelf?
Ї всегдђ кладџ егћ на пћлку. (клђсть Impf.) I always put it on the shelf.