You would be better off not using a passive construction when it is not present in the Latin text. Just say that they are leaving the horse. Everyone knows who “they” are. Plus, the sentence implies a broader context.
They are leaving the great wooden horse under the city gates at night.
I didn’t even realise that I was using the passive in my translation. I guess I just translated what I thought would make sense as opposed to what was actually written.
Sometimes it happens because we forget that in Latin the subject is often implied. On an exam using passive instead of active constructions may even cost a few points.
Magnum equum ligneum sub portis urbis Troiae nocte relinquunt.
A good clue that you will have an active verb is that there’s a 2nd declension masculine word ending in -um. That indicates accusative case. When there’s no proposition with it (as there isn’t here), odds are that it will be a direct object.
Right from the start of the sentence, you can say to yourself “someone did something to the big wooden horse”.