read · 3 min
This is the classic confusion. The deciding question: is the time finished?
Finished time → past simple
I saw her yesterday. · We went in 2019. (not “have seen yesterday”)
Unfinished / unspecified time → present perfect
I have seen that film. · She has written three emails today.
Signal words: yesterday, ago, last week, in 2020 → past simple. ever, never, already, yet, so far, today → present perfect.
More on the tense itself in When to use the present perfect.