Greg's right. The formal version of the word "you" (Sie) is always capitalized (in all grammatical cases, so "Ihnen" and "Ihrer" as well). Not so with the word "sie" meaning "they". In this sentence, that little distinction is your only clue that this is "you" and not "they".
But what if "Sie" for some reason came at the beginning of the sentence, and had to be capitalized? How could you tell the difference then?
Look at the verb ending, then context.
- Sie kommt = she comes
- Sie kommen = you (formal, singular or plural) come or they come
For the second sentence I gave, you need context to tell if "Sie kommen" means you or they. Duolingo never (99% of the time) has any context so they almost always accept both you and they as English translations.
If you were just given the audio, yes it could mean "they come" and it is an accepted answer. If you're given the written German sentence to translate, see Greg and Chubbard's answers.
Wondering the same thing. Or is it a special kind of sentence where the infinitive form is required?
Hmmm I had learned, at college, that "Sie" could be singular or plural even though it is always formal.
sie kommt, sie kommen, Sie kommen. she comes, they come, you come. I don't remember learning this (thought I could just look at verb), but I'm sure it's somewhere.