"You eat the apple."
Translation:Du isst den Apfel.
The capitalised 'Sie' means 'You' (formal).
The 'sie' can mean 'she' or 'they'. The way to distinguish between these two is to look at the verb. Sie trinkt Wasser. (she) Sie trinken Wasser. (they)
(Note: 'Sie trinken Wasser.' could also mean, 'You (formal) drink water.' We can always tell if it is the 'formal You' though, because it is always capitalised.)
Trick question: "ihr esse den Apfel" would, at first glance, look like a correct translation since one is focused more on the pronoun (you -> du, ihr, Sie). However, the verb essen is incorrectly conjugated here for ihr as "esse"; the correct translation would have to read "ihr esst den Apfel"