I'd say this is quite a philosophical question. 'Can' indicates possibility here. However, possibility merges with the realization of the action, that's why it's difficult to draw the borderline between: I saw a girl through the window. and I could see a girl through the window.
in past sentences use could and in present and future use can.
This isn't exactly true, Vjlenin. It leads ESLs to the incorrect assumption that 'could' is the past tense of 'can'. It isn't. In modern English modal verbs are tenseless. They carry modal meaning into sentences but they don't mark tense.
"I can see the house now."
"I could see the house yesterday"
You can't have seen the house yesterday. It was torn down a few months ago.
"I think I can see the house tomorrow too".
"I think I could see the house tomorrow too"
The two example sentences show that modals can be used in all situations, past, present and future.