How do I know this is a plural "you" (jullie) when the English can go either way?
You don't. Hence you can use all options, provided you use the correct conjugation.
I wrote "Waroom ga je niet" and it was marked incorrect
Problem solved-I wrote waroom instead of waarom
Why ga and not gaan?
gaan is plural (only for zij/wij/jullie)
If you use gaan, it must be with jullie.
If you use ga, it must be with je/jij.
What's wrong with "Waarom gaat je niet?"
In the form of a question, the "t" is dropped when it applies to "je."
Why is "Waroom ga je niet" incorrect here?
How do I know that "you" in this sentence indicates the plural "jullie" and not singular "je"?