别接受！

Congratulations! Just remind you that you have NO OBLIGATION to accept it on the spot.

Only say "thank you, and I will review your written offer as soon as I receive it."

Say whatever sweet words and try to figure out what the maximum $ you can argue for.

You may also be asked when you can start. (well, I don't know the specifics of your case, a general guess)

If your offer is given, then it is your turn to ask every question on your mind.

Your offer won't be pulled back for the questions you asked.

Surely it is good time to ask if GC is sponsored and what is the exact policy. But do not expect you can argue on this, it should be a standard, firm-wide policy which your manager cannot change. But he should give you a clear explaination of the policy.

上次和电话里他就问过我什么时候可以上班,我也不想早上,(很懒啦).但是他们知道我的OPT已经开始了.

Eariler to start working, earlier to receive paychecks.

可以问公司是否给办绿卡吗?这些我也不太懂.