Hm, I get why you'd say that. 'The tall girl is watching' -> Het lange meisje is aan het kijken, is what I would say. Or 'het lange meisje kijkt' -> 'the tall girl watches' Maybe it is because when you say 'zit te kijken', doesn't mean per se the girl is actually sitting...
For example, if I would be talking to a friend, and something happened behind the friend which would cause me to not pay attention to the friend anymore, the friend could be like 'hello? are you hearing what I'm saying? and I'd be like 'Oh sorry, I was watching X' -> 'Ik zat (past tense) te kijken naar X', while actually we are standing.
Well yes. I would just use it as it is. If I'd be sitting in front of the tv, I would use 'Ik zit te kijken', if I'd be standing somewhere I'd say 'Ik sta te kijken'. And if you want to be safe just use 'Ik ben aan het kijken' (I am watching).
When zitten is not sitting per se.. it's not usually like that. But it was my explanation for the difference between the english and dutch in the sentence from the exercise. Say it as is, and you're good in real life (:
There doesn't appear to be a "perfect" translation of Dutch --> English in this case, beyond saying "Yes, she's probably sitting but the emphasis is on the watching". Shouldn't Duo should accept answers involving sitting, since translation is going to be imperfect in either case?
Yes, but I would recommend saying "Het lange meisje is gaan zitten om te kijken" (when she is sitting already) or "Het lange meisje gaat zitten om te kijken" (when she is not yet seated, or at the exact moment she is seating down).
Besides that, "omkijken" means "looking back" (among others). So "Het lange meisje zit om te kijken" with the emphasis on "zit" means "The tall girl is seating to watch", but with the emphasis on "om" it means "The tall girl is looking over her shoulder".
No, she doesn't have to, but it's not totally meaningless either. It's a kind of implicative meaning. "Hij staat te dromen" or "Hij zit te dromen" suggests someone who is daydreaming, while "Hij ligt te dromen" suggests someone who lies sleeping, but this is not a strict rule.