Past Simple or Present Perfect?

Question

I have a problem using those tenses in practice, or even analysing them in books or movies.

For example : A character says "I opened the doors" (well the doors are opened so he and his friends see that they are opened)

But it should be used Present Perfect so the correct answear is "I've opened the doors" (result they are opened)

Next example can be "I bought a car" - He is with his friends but they result is that he still has this car but he used "bought" in Past Simple tense

But he could also use "I've bought a new car" same thing ... So what should a do about that ?

Answer 1

Both I opened the doors and I've opened the doors are grammatical. To decide which one you want to use, you need context. Sometimes, one or the other is the most natural, and sometimes both tenses work perfectly well.

The difference between the present perfect and the past simple isn't in how recently an action happened. For example, in the following two scenarios you might have opened the doors a minute ago:

I've opened the doors. You can go in now.

In this one, it is best to use the present perfect, because you're explaining how the action of opening the doors had an effect on the present.

I opened the doors, and all the dogs instantly ran out of the house.

Now, the action of opening the doors had the result that the dogs escaped. Since the action of the dogs escaping was in the past, it is best to use the past tense.

There are lots of times in English when both tenses work. The answer to your question about what you should do in this case is that you should choose one—it doesn't matter which.

Answer 2

The difference is really very subtle. Especially in spoken English, a speaker may use the simple past tense instead of the present perfect, depending perhaps on what they're most interested in conveying.

I opened the doors.

Here, you are relating what you did. It's almost as if you were telling a story. You may have opened the doors recently or a long time ago. They may still be open, or they may be closed.

I've opened the doors.

Here, you've opened the doors recently. The doors are still open as far as you know. It could be that this was something you were asked to do or something you had been planning to do, and you're informing someone that you've done it.

I bought a car.

You're informing someone of what you did, or relating a story. You may have bought the car an hour ago or ten years ago; it's not clear without further information.

I've bought a car.

Here, the point is that you now have a car. You didn't have a car before, and now you have one. Moreover, you've bought it recently; the last time you spoke to this person, you did not have a car. (If you said "I've bought a new car", this would change the meaning completely; it would mean you had a car, but you've recently bought a new one, either in addition to or as a replacement for your old car.)

Frankly, I think that the only way you can learn to distinguish between these two tenses is by reading a great deal. Read as many novels as possible, preferably well-written contemporary novels with a lot of dialogue. Over time, you will develop a feel for which one to use when.

Edited to add: The above is not a set of rules. Rather, it is an attempt to describe what a native speaker thinks when they hear the quoted sentences on their own, without any context. There is a lot of overlap between the two tenses if no context is given. In a given context, however, one of them may be wrong. For example, if you say, "Before, I used to walk to work, but now I've bought a car," you have to use the past perfect. If you ask someone, "Where did you go?" when talking about the previous weekend, you have to use the past simple. Since there is no hard and fast rule that can explain all this, I reiterate my suggestion that the best, and perhaps the only way, for a non-native speaker to learn to use these tenses correctly is to read hundreds if not thousands of books.