あなたが年をとったらわたしが面倒を見てあげます。

you (referring to someone of equal or lower status); dear (what a wife calls a husband)

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

year; many years; age; past one's prime; old age

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to harvest (a crop)

I; me; private affairs; personal matter; secrecy; selfishness

trouble; difficulty; care; attention

to see; to look; to watch; to view; to observe; to look over; to look on; to assess; to examine; to judge; to look after; to keep an eye on; to take care of; to view (e.g. flowers, movie); to try; to try out; to see that...; to find that...