Present Perfect: when and why
The present perfect connects the past with the present. It is one of the most useful tenses and one of the most misused.
Form
Use have or has with the past participle: I have finished, she has lived here for years. Questions invert the auxiliary: Have you ever been to Lisbon?
When to use it
Use it for experiences without a stated time (I have visited Spain), for situations that started in the past and continue now (We have known each other since 2019), and for recent events with a result in the present (He has lost his keys).
Common mistakes
Do not use the present perfect with a finished time expression. Say I saw him yesterday, not I have seen him yesterday. Compare for and since: for takes a period (for three weeks), since takes a starting point (since Monday).