I am confused about when to use finish instead of complete and vice versa.
May you help me in understanding when to use those words?
They are kind of synonyms but not really interchangeable.
To finish means simply bringing something (a task or activity) to an end, or simply stop doing it:
To complete means finish making or doing, such as in:
But another meaning peculiar to "complete" is making something whole or perfect, or with the meaning of "filling a form"::
While to finish means merely bringing something to an end, or stop doing that something, to complete has the acception of fulfilling something.
Lastly, I'll leave you with this I found on the net: