Present Perfect expresses the idea that something happened (or did not happen) at an unspecified time in the past; the exact time is unimportant. The sentences used by your teacher are correct, but you can also say, 'I lost my wallet a week ago' and it will be just fine.
If you say, 'I have smashed the window' it will most likely be understood that it happened very recently and you still work on fixing it up, as opposed to saying, 'I smashed the window' and having nothing to do with it any longer.
Present Perfect is very confusing, if you still do not understand, do not hesitate to ask more questions.
Originally Posted by Lenka