In your scenario, the $800 would've been deducted on line 14 of the 2017 Form 2441, reducing the 2017 line 15 amount to $4,200, and the taxable benefits on page 2 of the 2017 form would be based on $4,200 rather than $5,000. Then, in 2018, the $800 would be carried over to Form 2441 line 13, which would increase the 2018 line 15 amount to $5,800, and the taxable benefits on page 2 of the 2018 form would be based on $5,800 instead of $5,000.
