"I was the only one there."
Translation:Tôi là người duy nhất ở đó.
Is this translation correct? Why is it in the past and not the present tense?
Well, because you can't be "there" at the moment you say that, unless you somehow can exist at two location at the same time.
Common sense really, you're "here" right now (wherever that may be for you) and not "there".
This must be the first time I've seen "common sense" applied to a duolingo sentence. Good to know the reasoning.
For starters the way tense works in Vietnamese doesn't always require you use a particle like đã/đang/sẽ. In this case the word đó means "there", away from your current position. It just makes sense to treat it as being in the past tense. You are "here", not "there", however, it's possible for you to "have been there" or "to be there (in future)". Immediately we've ruled out it being the present tense so we're left with either the past or future. For the future you'd have to use sẽ while the past action does not require đã.