If you are single, no interest income , no mortgage interest is simple tax filing which is used 1040 ez .
If you are married, dependent child, mortgage interest, you got stock from work and sold or you bought and sold stock on those years or have income from self-emplyment income, you bought house and sold it all these things are considered complex or complicated.

If you live in one state and work in another state , you may have to file state tax for both state which is not cosidered simple. For example ,  filing VA  3013, moved from RI but worked in MA in 2012. He has to file state tax for VA,  RI as well MA.
Even you get refund if you have filed :  IRS may send you a letter stating you owe this much. Then you need to go to a tax filing office and figure out wheter you owe or not.

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