does it matter very much when you use simple past instead of present perfect?

Answer 1

Yes. Simple Past is a statement about something that has happened. Perfect Present is something that happened at some point in time and is still happening OR a statement about something that is of no specified date and no longer happening.

Example: Would you say, "I was born in 1900" or would you say, "I were born in 1900." ? You were playing in the mud when you were a child. Let me find a website that explains it better than I am and I'll edit my answer and post it here.

Found one with many examples and explanations: http://www.englishpage.com/verbpage/presentperfect.html