The key rule

TenseUse when…Example
Present Perfect
have/has + pp
The time is unfinished or unspecified. The action is connected to now. We don't say exactly when. I have seen that film. (ever in my life)
She has lived here for years. (still now)
They have just arrived. (very recent)
Past Simple
verb + -ed / irregular
The time is finished or specified. The action is completely in the past. We say or imply when. I saw that film last week.
She lived in Paris from 2010 to 2015.
They arrived at 6 pm yesterday.

Signal words

Present PerfectPast Simple
ever never already just yet since 2020 for 3 years recently so far today / this week yesterday last week/month/year in 2010 ago when I was young at 3 o'clock then / at that time
I have been to Rome. → past simple follow-up: When? → I went there in 2019.
today / this week: Use Present Perfect when the time period is still in progress: "I have drunk three coffees today." (the day isn't over). Use Past Simple when the time is over: "I drank three coffees yesterday."

Exercises

Present Perfect or Past Simple?

1. She ___ to London last summer. (go)

2. I ___ never tried raw fish. (have)

3. ___ you see the match last night?

4. This is the best pizza I ___ ever eaten.