I was - byl jsem tam a už nejsem; I have been - pořád tam jsem, např. I have been in the library for 2 hours (jsem tam 2 hodiny).

I have been in the library - It is used when you tell someone that you have visited the library before.

Používá se, když někomu řeknete, že jste knihovnu navštívili již dříve.

I was in the library - It is used when you answer a question asked by someone. For example : "Where were you ?" And you answer "I was in the library."

Používá se, když odpovíte na otázku někoho. Například: „Where were you?“ A vy odpovíte: "I was in the library."

Záleží na kontextu :) ...

I have been to the library (právě odsud odcházím, nebo jsem ještě tady)
I was in the library... Byl jsem tam, ale už nejsem.

Děkuji :)

I have been-jiz nekdy jsem byl,I was-byl jsem tam-treba vcera...

Have been znamená že v knihovně byl a nevíme jestli ten dej už skončil. I was znamená že v té knihovně byl a už tam není. Ten dej se dokončil.

Hi Jan,
'have been' shows the result or experience and NOT used with past tense expressions:
- I have been to Italy (means that during your lifetime you visited Italy, but when does not matter, the focus is on your experience)
'was' shows exact time in the past, so it's usually used with past time expressions:
- I was in Italy 2 years ago/yesterday (here we can see past tense expressions like AGO/YESTERDAY, which means the focus is on when we did that exactly in the PAST)

NOTE:
The differences between Past Simple and Present Perfect don't end just with the point I explained above. I could recommend you a book to review more about it: "English Grammar in Use 4th or 5th edition"

Good luck in your learning process :)

I have been = you are either still in the library at the moment of speaking, OR you being in the library has an effect on the present, at the moment. 'Have been' has an unfinished feeling.
I was = means I was there, that's it, that moment is gone and closed, it has no effect on the present moment or moment of speaking.