"You do not have children."
Translation:Tu nu ai copii.
This sentence also had the correct translation, "Tu n-ai copii." I am new to Romanian. Are there lots of other contractions that use hyphens? This was not taught on duolingo before I was tested on it.
There are. Contractions are crucial to speech as they greatly speed it up. Some of them are also mandatory.
I wouldn't dare to try and explain them, because I'll surely miss a lot.
Relevant to this question, "nu" before any compound perfect verb can become "n-":
eu n-am văzut - I didn't see
ei n-au văzut - they didn't see
voi n-ați văzut - you (plural) didn't see
This page is a good start; it shows how unstressed forms of the pronouns in all cases get contracted to other pronouns and to verbs and also how they get contracted to imperatives and gerundives. This should cover a lot of the contractions you may encounter.
There's also this wiktionary page that lists some common contraction which is useful if you want to know their meaning + origin.
This isn't a perfect verb in this case, it is present tense. N-ai should not be used here.
Its part of the learning process. Having to find out why you were wrong helps you to remember the rules .
both is correct: "tu nu ai copii" and "voi nu aveti copii".
no correct:"tu nu aveti copii" and "voi nu ai copii". there must be concondarcia in the conjugation of the verb
English "you" can be either singular or plural, and there's no context here with which to tell. I don't see why "Voi nu aveți copii" shouldn't be accepted here so long as you conjugate it correctly.