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Did you get it Alex? Cause you didn't reply or anything!
For your questions:
1. It was me (or I?) who opened the file. --» It was "I" who opened the file. [I opened the file✔- Me opened the file✘]...

Well Alex, i hope this will help you a little ;

-It was I who got a present last night.
Here "I" is subject. To understand it better, note that you will say "I got a present last night."
You...