Depends on the question. Talking about the past What did you do when your bus service stopped last year. I bought a car. Talking about now and recent past I hear your bus service has been stopped, what have you done about it? I have bought a car. Talking about now, the future and the past. I hear your bus service is being stopped, what are you doing about it? I have bought a car.

I have bought a car. <- Present Perfect & I bought a car. <- Past Indefinite

In my understanding, there is an implication in the sentence "I have done sth" that this "sth" has exerted an influence on you. for example, "I have bought a car" may be followed by "so I can drive out today" ,and the past tense only emphasizes on "sth" itself ,or maybe on when and where it happened. By the way , I'm a newbie in English. Please point it out if there is anything wrong

technically speaking, there is no real difference between the 2 sentences. They both refer to the same thing and both have exactly the same meaning. However the use of the word "have" i guess relates to ownership. for example "i HAVE a pet" or "i HAVE a car". In my understanding i believe the use of "have" in your sentence makes me think of a past action, where as the second sentence makes me think of something you have done quite recently within the last hour or so.