If I forgot something on last year's return, can I put it on this year's?
Unfortunately, that's not how taxes work.
You're only allowed to claim credits and deductions in the tax year they apply to, and income needs to be reported in the tax year it was earned or received.
For example, if you made a sizeable charitable donation in 2015 but forgot to include it on last year's return, you can't claim it this year because the donation occurred in 2015.
Likewise, you can't report income you received in 2015 on your 2016 return, nor vice-versa for that matter.
Either way, if you already filed the prior-year return for the tax year in question, you will need to amend that return.