如果我在圖書館溫習，忽然見到老師。佢問我做緊乜…我應該點答為佳？

(this is what i think only)
first of all, are you revising his/her subject? if yes, then you should pretend to be thinking very hard on one of the theories or questions, then tell him/her you are revising so hard on his/her subject because you do not want to disappoint him/her. I think then he/she will ask you what don't you understand. So it would b better you think of an intelligent question to ask so as to impress him/her. If your result are not so good in his/her eyes, then you should ask some basic questions so he/she can see your effort to work hard on his/her subject.

If he/she is not your subject teacher, you should even act revising harder and seems to have more difficulties than you actually are. Then i think she/he will ask if this subject is very difficult to handle, then you should say yes and show your face as "how come my hard work doesn't pay off" that disappointing look, then she/he will get sympathesized by you. probably, he/she will have a good impression on you and tell your subject teacher you are a good student.

你應該如實作答，告訴他/她你正在溫習。

我正在温習，上次測驗/考試成績不太好，想努力d，希望下次考得更好。老師，你會支持我嗎？

應該話：我而家係到搵緊書，老師你係咪有事搵我呀？

吓~ 你咩照直答係度温書lo, 你又唔係係度做啲犯法野, 做咩要自己攞來煩呢??

我覺得你應該話俾佢聽:X老師早晨/午安,我係度溫緊習呀!你呢?

米直接話佢知你響度溫緊書囉，老師聽到都高興啦你咁勤力！

Tell her the turth! She will think you are a good student!

2006-12-08 15:56:36 補充：
Don't worry! 如果u在圖書館溫習,I also think u are a good student!

應該答:我在圖書館溫習!