You mean the "át"? It is not a preverb here. It simply means "through", it is a postposition of the noun "ablak(on)".
If you want to say "I see" like this in Hungarian, say "I understand" instead: Értem.
"Látok" is a verb itself. I am not sure I understand what you mean. There is no preverb in this sentence, therefore there is nothing to be separated.
But in this case he meant the verb " átlátni = to see through something " and the preverb át got separated and moved to the end of the sentence.
It is misleading. átlátni exists, but in this sentence, there is no separated "átlátni" , but rather látni is the verb, and át is just a postposition.
where is the "you" in this sentence? I read it as "I see through the window". What i see is not specified. Or is it? Should not there be "teged" somewhere as in the other sentence in this section?
-lak / -lek is a special verb ending sometimes called the "I->you" conjugation. It indicates that the subject is first person and the direct object is second person.