What is the difference between simple past and present perfect tense?

Simple past describes an action completed at a definite time in the past, often with a time marker: I finished my homework yesterday, I finished it at 5 pm. Present perfect connects a past action to the present and is used when the exact time is unknown or unimportant, or the result still matters now: I have finished my homework (so I am free now). A clear signal: words like yesterday, last week, in 2020, and ago take simple past, while just, already, yet, ever, never, since and for take present perfect. So you cannot say 'I have finished it yesterday'; with 'yesterday' you must say 'I finished it yesterday'. Match the time word to the tense and you will not mix them up.