How do I know when to use the perfective or imperfective aspect of a verb?

While the tense of a verb describes when an action takes place in time, the aspect describes how it takes place (whether it was done repeatedly or once, whether it is an ongoing process or a sudden action, etc.)Generally speaking, the present tense is always formed using imperfective verbs. For example: Маша кушает яблоко (Masha eats an apple).Here the verb кушать is imperfective (as opposed to the perfective form покушать)Otherwise, the imperfective aspect is used in several other situations: When an action is repeated an unspecified number of times. For example: он читает каждый вечер (he reads every evening)When an action is ongoing/ unfinished. For example: она спала когда он вернулся домой (she was sleeping when he came back home)When the verb denotes a state rather than an action. For example : он жил в маленьком доме (he lived in a small house)When there is no action. For example: я к доктору не ходил (I did not go to the doctor (so basically nothing happened) )When an action has been attempted but without result. For example: я тебя ждал на улице (I waited for you outside (but you never came) )When you want to express duration: он весь вечер смотрел телевизор! (he was watching TV all evening!)The perfective aspect is used in other specific situations:When it is a single action with an achieved result. For example: он помыл руки (he washed his hands)When it is a single action or event with context. For example: Вчера я сходила на рынок (I went to the market yesterday)When you talk about sequences of completed actions. For example: она позавтракала,почистила зубы и умылась ( she ate breakfast, brushed her teeth and washed her face)