Draw the line exactly

In A2 the rule was simple: if the time of the event is named — Past Simple; if not — Present Perfect (A2 #20). It is true, but it does not answer the main question: what do you choose when no time is named and both seem to fit? Here is the whole border.

The form — a reminder:

The main difference is where the speaker looks from. The Present Perfect looks from "now": the event was in the past, but the talk is about what has come of it now. The Past Simple puts the event in the finished past, on its own shelf of time, and the talk is about the event itself. So the question "and what now?" fits the perfect, and "when was it?" fits the simple past.

When you choose the Present Perfect.

The result can be seen now: I've lost my keys — I have no keys, I can't get in. Compare: I lost my keys last week, but I found them — the episode is closed, and there is no result.

News. The first report of an event is almost always in the perfect, and the details are in the Past Simple: the details bring a time and a place, and the perfect cannot stand a time.

The period has not finished yet: I've had three coffees today. If the period has finished, you need the Past Simple: I had three coffees yesterday. this morning is divided in the same way: at ten in the morning — I've drunk two coffees this morning, in the evening — I drank two coffees this morning.

The experience of a person who is alive, or work that can still go on: She has written four novels. About someone whose life has ended — the Past Simple: Dickens wrote fifteen novels.

Time words help, but they do not decide everything:

never, recently, today, this week go with both tenses, and the same thing decides: has the period finished? I've never been to Wales — life goes on; I never saw him again — the story has been told to the end.

British and American. In American English the Past Simple is often used with just, already, yet: Did you eat yet?, I just saw him. This is not an error, but in British English, which we are learning, people say Have you eaten yet? and I've just seen him.

Compare:

How to understand the other person's choice. The tense of the verb tells you what the person expects from you. If someone says I've lost my keys, they are reporting a problem and most likely expect help or sympathy. If someone says I lost my keys, they are starting a story, and details will follow. The question Have you seen Tom? means "I need him now", while Did you see Tom? is about one particular occasion, for example yesterday's party.

Don't say this. ✗ I've been in London last year → ✓ I was in London last year. last year closes the period, and the perfect is impossible with it, even if the trip is part of your experience. For experience without a time — I've been to London (A2 #4).

Check yourself. You are looking for your glasses and say to a friend — I lost my glasses or I've lost my glasses? Why can't the question When did you buy it? be in the perfect? It is evening, and you are telling someone about this morning: I've had or I had two coffees this morning?