"I was the only one there."
Translation:Tôi là người duy nhất ở đó.
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.
To echo that, in my experience, more often than not Vietnamese does not use đã or sẽ when speaking of the the past or future. DL emphasizes them, so we learn them, but in everyday life most often you just use the verb by itself, and you can tell from context whether it is referring to the past, the present or the future.