What is the difference between the perfective and the imperfective aspects?

In Russian there are two aspects the perfective and the imperfective, they can both be used in the past tense, the imperfective is also used for the present tense and the perfective for the future.In the past tense the aspects are generally used to express the difference between completed and non-completed actions:The imperfective is used when describing habitual or repeated actions (used with adverbs such as always, usually, often), unfinished or continuous action or when there is emphasis on the process of an action (for example in a sentence such as, in the evening I like to read a book, вечером, мне нравится читать книгу) The perfective is used to show single, completed actions (я прочитал книгу- I finished reading the book), a series of events in the past, the start of an action or and action performed for a limited period of time.