How do imperfect and perfect verbs differ from each other in meaning, tense and use?

Perfective verbs mean an accomplished action, the result or the end of an action. The answer to the question is what to do? (look, add). Such verbs can only be put in the past (what did you do? Looked, added) and future tenses (what will he do? See, add). Imperfective verbs answer the question what to do? (write, watch) and mean a long, unfinished action. Such a verb has all 3 tenses: the present (what is he doing? Writes, looks), the past (what did he do? Watched, wrote) and the future (what will he do? Will look, will write). The future tense of imperfective verbs, in contrast to the future tense of perfective verbs, is formed using the word “to be” (I will look, I will write).