"You do not have children."
Translation:Tu nu ai copii.
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.