You can say "in here" when you enter a room or building, that is somewhere inside. "In here" is more specific. By just using the word "here", you can be talking about a general location, not usually inside but it could be inside. Compare: - It's cold in here (you've just entered a building). - It's cold here (in Russia).

generally refers to being inside of a building or enclosed space when you use "I was in here" "I was here" is more broad and could cover a very large area.

I'm also a student and my English is not well enough. I might be mistaken but it seems to me that "I was in here"is not right grammatical combination. May be "I was here in..."?

I was in here is not really used in a normal conversation. But I was in here implies that you were inside something like a box. I was here is what you want to use as a standard for past tense of having been at a particular location at some point in time.

When you say ' i was in here' , your meaning is that you are inside something eg room , building , etc and you want to emphasised that fact. "I was here" is telling someone you were here somtime in the past , without emphasising whether you were inside , outside , under . above or around here. Just here .