It's my sixth year on an F1 visa (I was a student for 5 years up until June of 2010, am now teaching on an OPT), so I can no longer file as a non-resident (according to CINTAX, a tax preparation software for foreign students and scholar).
I received both a W2 (for the months that I taught) and a 1042-S (for the months I was a student). The 1042-S has income code 19, and the gross income is below the $5000 limit covered by the tax treaty, so I expect it to be tax-exempt, but where do I put it on the 1040EZ form? Do I have to add any extra forms? Do I have to do a 1040 instead?

If it is STILL tax-exempt (and THAT depends on the tax treaty), you list the income as wages on Line #7 of Form 1040, then list the same amount on Line #21 as a NEGATIVE number, then submit Form 8833 that must be MAILED with the Form 1040 to explain the treaty exemption.

If this seems a bit much for you to handle, I do this kind of return routinely, so if you need professional help filing your return, email me at [email protected].

Thanks!
Now, line 21 says "Other income. List type and amount" and then has a blank space for me to fill out: what should I write there?

Ah, found it! "Exempt Income: Tax Treaty Portugal Art. 23"

From http://www.irs.gov/businesses/small/international/article/0, id=96438,00.html , I do not have to file Form 8833 "for claiming a treaty exemption that reduces or modifies the taxation of (...) income of (...) students (...)"; and furthermore "for payments or items of income that are otherwise required to be disclosed total no more than $10,000". Right? (Filing taxes is hard, but even harder is feeling sure that I am doing it right... )

And then the remaining question is if indeed I can still claim the tax treaty. The tax treaty is good for five years, which having started in August 2005 should end August 2010, and I am trying to apply it for income from the first 6 months of 2010. Does that sound right?

You should submit the Form 8833, just to be safe.

For the F-1 visa, a PART of the calendar year counts as a full year. If you arrived in 2005, then 2009 was your last year as a non-resident alien. For Tax Year 2010, you are a resident alien.

Yes, I am going to file as a resident (form 1040), the question is if the income that I received in the first half of 2010 (which was within 5 years of my arriving in the US) and that shows up on the 1042S (the income for the second half of the year is all in W2s) is still under the tax treaty. Do you know how 5 years are counted in this matter? (If like you say it is calendar year, then my first-half-of-2010 income is not under the tax treaty).

Thank you so much for your advice so far!

Not sure; I would have to research the tax treaty, which, at THIS time of year. I will NOT do without compensation.

I completely understand that! Thanks though :)