Of all the choices English grammar asks an intermediate learner to make, this is the one that keeps going wrong long after the rule has been learnt. Both the past simple and the present perfect describe something that happened before now, both are taught in the same week, and the textbook explanation, that one is finished and the other is not, collapses the moment you meet a present perfect sentence about something plainly over. The rule is real, but it is usually stated backwards.
What each tense actually claims
The past simple is formed from the past form of the verb alone. The present perfect is formed from the auxiliary in its present form plus the past participle, and that auxiliary is the important part: it is a present tense verb. That is not a piece of trivia, it is the whole difference. A present perfect sentence is grammatically a statement about now that happens to mention a past event. A past simple sentence is a statement about then.
I lost my keys.
I have lost my keys.
Both lines above describe the same event, and both are correct English. The first places that event in finished time and says nothing else; it invites a follow-up question about when. The second says that the event has a present consequence, and the consequence is the point: the keys are still missing now. Speakers choose the second when the present result is what matters, and the first when the event itself is what they are reporting. Neither is more polite, more formal or more advanced than the other.
The one question that decides it
Here is the test that works in practice, and it is not about whether the action is finished. Ask instead whether the sentence locates the event in a finished stretch of time. If it does, the past simple is not merely preferred, it is obligatory, and the present perfect becomes ungrammatical.
I have seen that film last night.
I saw that film last night.
I have seen that film.
The first line above is the single most common error at this level, and it is a hard error rather than an awkward phrasing: a native speaker will not produce it. It fails because the closing time expression names a stretch of time that is over, and a tense whose auxiliary is present cannot sit inside finished time. The second line repairs it by matching the tense to the time expression. The third repairs it the other way, by removing the time reference altogether, and now means something different: it reports an experience without saying when, which is exactly what the present perfect is for.
The time expressions that force your hand
Because the choice is decided by how the sentence frames time, a small set of expressions effectively makes it for you. Learning these is more efficient than learning the abstract rule, because they appear in most of the sentences where the choice arises.
yesterday, last week, in 2019, two days ago, when I was a child
already, yet, so far, up to now, recently
today, this week, this year, ever, never
Every expression in the first line above names finished time and requires the past simple. Every expression in the second line points at the stretch running up to the present moment and requires the present perfect. The third line is the interesting one, because those expressions can take either tense: the period they name includes the present moment, so the speaker decides whether to treat it as still open or already closed. That is a genuine choice rather than a rule, and it is the reason two fluent speakers can describe the same morning differently.
Why your own language is working against you
If you speak French, Spanish, Italian, German or Dutch, you have a compound past form built the same way as the English present perfect, from an auxiliary plus a participle. In several of those languages that form has become the ordinary way to talk about finished events in conversation, and the older simple past survives mainly in writing. The shape maps across perfectly; the meaning does not.
So the error is not carelessness, it is a well-formed sentence in your first language translated word for word. It produces present perfect sentences everywhere a Romance or Germanic speaker would naturally use their compound past, including inside finished time, which is precisely where English forbids it. Speakers of languages with no perfect construction at all, including Slavic and most East Asian languages, make the opposite error and under-use the present perfect, reaching for the past simple even when the present consequence is the whole point.
Identify which of the two you are, because the correction is different in each case. If your language has the compound past, your task is to stop using the present perfect by default and reserve it for present relevance. If it does not, your task is to notice the situations where English expects it and use it deliberately.
Experience, duration and the unfinished present
Three situations account for most correct uses of the present perfect, and all three share the feature that the present moment is inside the picture.
She has worked here since March.
Have you ever broken a bone?
They have finished the report, so we can send it.
The first line above measures a stretch of time that began in the past and is still running: she works here now, and the sentence would be wrong if she had left. The second asks about experience at any point in a life still being lived, which is why this pattern is so common in questions to people you have just met. The third states a past action purely for its present consequence, and the second half of that line makes the consequence explicit. In all three, replacing the tense with the past simple would either change the meaning or make the sentence false.
The related form that adds the continuous, covered in the lesson on the continuous version of this tense, keeps the same present connection but shifts the attention from the completed result to the activity itself.
The mistakes that cost marks
Four errors account for most of the damage in exam writing and speaking. The first is the one shown above: a present perfect sentence carrying a finished time expression. The second is using the past simple for a situation that is still true, which quietly tells your reader the situation has ended. The third is the wrong participle, since the irregular verbs are exactly the frequent ones and the participle is often not the same word as the past form. The fourth is dropping the auxiliary in speech, which turns the sentence into a past simple with an unusual verb form and undoes the choice you correctly made.
The third of those is worth attacking directly, because it is vocabulary rather than grammar and can simply be learnt. The reference list of irregular past forms gives you the participles beside the past forms, which is the comparison that matters here. If the regular endings themselves are still slowing you down, the lesson on building the regular past covers the spelling, and the article on pronouncing that ending handles the sound.
What to do this week
Stop asking whether the action is finished, because that question does not discriminate. Ask instead whether the sentence names a finished time, and let the answer choose the tense for you. Then take your own recent writing and look only at the sentences containing a time expression: the errors cluster there, and you can usually fix a week of them in ten minutes. When you want to test the choice rather than read about it, the practice set that makes you pick between the two is built on exactly this decision. Everything else on this site is sorted by level on the page that groups material by level, and by topic on the page that groups it by subject.