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I've bought a jacket. - I'm focusing that I have the jacket now and I'm telling that the action has some importance to the present.
I bought a jacket yesterday. - Does this mean that the action doesn't have some relevance to the present and that the sentence isn't as important as when I use present perfect? Does once we mentioned a specific time a sentence loses its importance? Thanks. I'm talking about the same jacket.
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I see you're still banging your head against a wall. Emotion: smile

So am I. Emotion: smile
whatchadoinI bought a jacket yesterday. - Does this mean that the action doesn't have some relevance to the present
Everything you tell others about yourself has relevance to the present unless you're just babbling nonsense. For the purposes of verb tenses the phrase "current relevance" ("relevance to the present") doesn't really mean that. Sometimes I don't know what it means myself, but if you keep reading, you may find some clues.

In any case for this kind of sentence, which I call "a simple, isolated fact", all you need is the simple past. I bought a jacket. I bought a jacket yesterday. I had two eggs for breakfast. I went to the post office. I played football. I listened to some music.

You need a more complex situation to justify the present perfect. The present perfect is almost something removed a step from the simple, isolated facts. It almost says that 'the act of something happening' happened. The focus is not on the act itself (That's the simple past) but on the fact that it happened. (If you know calculus, think of it as the derivative of a function.)

I have bought a jacket is sort of like The act of my buying a jacket happened, i.e. This —> I bought a jacket <— happened.

I have gone to the post office is sort of like The act of my going to the post office happened, i.e., This —> I went to the post office <— happened.

Often you only want to focus on the fact that something happened when it's part of a larger series of actions which are all intended to happen. Suppose you were putting together some kind of sports outfit. You need the shirt, the pants, the jacket, the cap, the shoes, .... (Now of course the people you're talking to have to know that that is your plan.) Now you can say, I've bought a jacket, and they'll ask, Have you bought a shirt yet to go with it? Or they'll say, Well, that's the first step. Now all you need is the shirt, the cap, .... So when you say I've bought a jacket, your focus is that I bought a jacket happened — within a series of things that are expected to happen. It's like an update, like a progress report. It says where you stand now within a whole project.

Of course that's not the only way the present perfect works, so don't try to make every case of the present perfect fit into that mold.

CJ
Comments  
whatchadoinI've bought a jacket.
Usually, if we must use present perfect to express past to present relevance as your sentence, it is advisable to add " just" after "have". For instance: I've just cleaned the floor, take your shoes off!. This in essence says, I spent the time " started in the past, maybe 2 hours ago and I just finished the cleaning as you walked in. Is the present a great importance, I say not. We can simply just use the simple past tense: I just cleaned the floor, please take your shoes off.

As for your "jacket " sentence, if you wife just came home from shopping and she can't wait to pull the jacket out from the bag to show you. She may say : I've just bought this jacket; what do you think? I think you are so caught up on present perfect right now that you are actually creating confusion for yourself.
whatchadoinI bought a jacket yesterday. This is fine. - Does this mean that the action doesn't have some relevance to the present and that the sentence isn't as important as when I use present perfect? You may say that. But the better way to explain it is that, the time you bought the jacket was yesterday and there is no relevance whatsoever to the present conversation.
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 CalifJim's reply was promoted to an answer.
Thanks! And I'll know when I have to specify a time because I'm the speaker, won't I?
whatchadoinAnd I'll know when I have to specify a time because I'm the speaker, won't I?
Hopefully. If you're paying attention to what you're saying!

CJ
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