Hi Lara. It depends on if you are talking about a specific time in the past, or about something that happened in the past but not at a specific time. For example. 1." I read the book." 2. "I have read the book." As you can see both means that you are done reading the book and it happened in the past. Number 1. is the simple past and it means that you read the book at a specific time like last week. Number 2. is the present perfect and it means that you read the book at an unknown time in the past, maybe last year or even three years ago. I hope this helps.

It's a matter of focus. The simple past focuses on the past action itself: "I took the Unit 3 test this morning." Here, you are talking about the event of taking the test. You might go on to describe the experience of that further: it took you three hours to finish; you found the vocabulary section easy, but you don't think you did well on the grammar; your brain was tired afterward, so you ditched the books and went for a long walk; etc. The present perfect focuses on the present time, even though the event itself happened in the past: "I have taken the Unit 3 test." Here, you are talking about the fact that the test is currently behind you. You might go on to describe the effects of that further: you feel a load off your mind now; your teacher should be grading the test; it's time to turn your attention to studying Unit 4; etc. It might help to think of it this way: the simple past is simply about the past, whereas the present perfect is about the present effects of a perfected (in the sense of "completed") action.