Pop quiz! Fastest correct answer gets 10 points! Do you know...?

OK folks, let's play a little game! I'll ask a question and the first answer that I consider correct, clear and complete gets 10 points. Even if there's a better answer afterwards, the fastest one still gets the 10 points. But the answer HAS TO BE complete and clear. (Of course I already know the... show more OK folks, let's play a little game! I'll ask a question and the first answer that I consider correct, clear and complete gets 10 points. Even if there's a better answer afterwards, the fastest one still gets the 10 points. But the answer HAS TO BE complete and clear. (Of course I already know the answer. XD)

OK, those are the basic rules. You can do your own pop quizes too, if you want, with more interesting questions... :P

Can anybody tell me what's the difference between Simple and Perfect verb forms? For example:

"I saw." / "I have seen."

"I was seen." / "I have been seen."

Can both forms be used as synonyms in any case, or are there any particularities which we have to pay attention to?

Be quick about it... ;)
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