I have a question on behalf my friend:
He came to US in 2004 with F1 and work in OPT in tax year 2005. He should have filed 1040NR. However, he carelessly filed 1040 for resident alien and already got tax refund from IRS. In this case, should he file amendment? What will happen if he should but doesn't file amendment?

He should file an amendment. The tax will probably be the same.

If he fails to amend the return, it is likely nothing will happen. However, if he wishes to file some form (say Form 843) that depends on him being a non-residnet alien, he will have problems.

Thanks. He paid over $10000 tuition in 2005. Can he claim education credits or education deduction as a non-resident? If he filed 1040NR, which line should he put $5000 treaty exemption?

No, he cannot claim any education credit as a non-resident alien.

You claim the treaty exemption on Line #22 of Form 1040NR.

Thanks a bunch. I may have more questions in the future.

I'll be here!