What would be the right way to say "The students had not eaten"? DL did not accept 'had' in place of 'have' so I think I used the wrong word.
Yup...."Have" is present tense, not past!
(I've only recently learned this while training to be a language teacher thanks to duo!)
That's right, but "The students have not eaten", despite using "have" (present tense) is really talking about some time before the present (it's called the present perfect tense). In fact, this sentence could be translated by the simple past too: "The students did not eat".
Hmmm.. I wonder why DL doesn't expand the verb conjugation charts now that we are learning past tenses?
So this could mean both "The students did not eat" and "The students have not eaten"?
Again, the English is in the present perfect – "have not eaten" – rather than the preterite "did not eat". Is Portuguese more like, say, French or Italian in this respect – where the passé simple or passato remoto are literary forms – rather than English or Spanish were the distinction is important?