finished action vs unspecific time in the past

Question

I know there are several rules when to use past simple or present perfect. for example specific words like :ago,yesterday,since,for,yet etc and in such cases is simple for me what tense to use but there are also other rules for example unspecific time and in this case we have to use Present Perfect

1) "Look,I have bought a new car"

but on other hand there is a rule for past simple - finished action.So if I say:

2) "Look,I bought a new car" - would it be incorrect?

If I dont say when I bought the car do I always have to use the Present Perfect even the action is finished? This two rules make me confused cause the action seems to be finished for me but without specific time and I dont know which tense to use

Answer 1

Both are grammatically correct, but they mean slightly different things and would be used in different circumstances.

Look, I bought a new car. [simple past]

You are declaring that you bought a car at a specific time in the past. You are probably showing the car in question to the listener.

Look, I have bought a new car. [present perfect]

You are declaring that you have bought a car at some point in the past. If you're talking about a specific car that you bought at a specific time, this form would sound awkward and maybe overly formal or antiquated (but the listener would still understand).

There are some more examples of the difference at VoA's Everyday Grammar column.

Answer 2

Simple past is normally used with “additional” information or details about when it happened even if it is not very specific. Example: I lived in Europe in my childhood”. If you use a present perfect tense you basically talk about some action in the past without giving a specific time frame. The reason for not talking about when it happened might be different. Example: I have seen this movie (already). You want to convey your “feeling” or “experience” much more than the time you did it. Basically, you often don’t want to see the same movie again; it doesn’t really matter if you saw it last week or two years ago. Your main “motivation” is not the time but the fact itself.