Present Perfect vs Past Simple: The Simple Rule
Learners mix these two tenses more than any other pair. One question settles it every time.
Past simple describes a finished action at a finished time: 'I visited London in 2015.' The time frame is closed, so the verb is closed too.
Present perfect links the past to now: 'I have visited London.' We do not say when, because the point is the experience, not the date.
The test question: does the sentence carry a finished time expression — yesterday, last year, in 2015, when I was a child? If yes, use past simple. If not, present perfect is usually safe.
Watch the signal words. 'Since' and 'for' pull towards present perfect ('since 2015', 'for three years'). 'Ago' pulls towards past simple ('three years ago').
Practice tip: write five sentences about your week with past simple, then rewrite them with present perfect and notice how the meaning shifts from event to experience.