If I forgot something on last year's return, can I put it on this year's?

No, 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 received.

For example, if you made a sizable charitable donation in 2023, but forgot to include it on last year's return, you can't claim it this year.

Likewise, you can't report income you received in 2023 on your 2024 return.

If you already filed the prior-year return and need to make these kinds of changes, you'll need to amend your return.