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American Opportunity Credit requires  you to be  in (a) 4 years of post secondary education -- not grad school; (b) have a Tax id  ; (c) no felony  and a bunch of other requirements.   

You came here on F-1 in 2012 and therefore started counting days present in 2017.  Therefore I suspect that you are either in grad school in 2018  or post doc  in 2018.  Therefore you are not eligible for American Opportunity Credit --- your 1098-T should have the grand school box checked -- perhaps you did not enter that and TurboTax therefore assumed  you are an under grad.

Does this answer your query ?

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@monsa , since for the years 2017 &  2018, you were a resident for tax purposes ( being taxed on your world income ), and had a TIN ( SSN or ITIN ), you should qualify for the American Opportunity Credit -- it is available  for a max of  four years

The " four years" is used in two  different requirements  ---(a) first four years of post secondary education - education year classification  , not duration      and  (b)   Not have claimed AOTC or Hope for more than 4 tax years.      Below is quote from the IRS:

"To be eligible for AOTC, the student must:

• Be pursuing a degree or other recognized education credential
• Be enrolled at least half time for at least one academic period

  *

  beginning in the tax year
• Not have finished the first four years of higher education at the beginning of the tax year
• Not have claimed the AOTC or the former Hope credit for more than four tax years
• Not have a felony drug conviction at the end of the tax year  "

Hope this closes your query / worry --- Good Luck