What is the difference between present and imperfect tense?

Imperfect tense is usually called past simple.

Past simple is used to talk about an even that happened, within a time frame, in the past and is now completed. -- I studied English last year.

Present tenses talk about

things that happen again and again -- I study English.

things that are happening now or around now -- I am studying English.

things that started in the past but continue now -- I have studied English for 3 years