tell me please what's the difference between "I have been to London" and "I was in London"
Visit the sites that Peachey recommended. All your questions will be answered there.
The first case " I have been to London " it means that I traveled to London for a while and returned.
The second case " I was in London " it means that my stay was in London just in th the past.
Thanks, Peachey. But I don't understand in what context I should use the firs form and in what context the second one?
It's the difference between present perfect (have been) and past simple (was). There's plenty of information out there, so I'll just link you: