"We have a child."
Translation:Vi har et barn.
The tense of the verb. "ha" is used with "å" (as in "å ha") to mean "to have", while "har" is present tense, like in the example.
It depends on the grammatical gender of the word after it. In this case, "barn" is neuter so you use "et". If the word had been masculine (e.g "hund" (dog)) you would say "en", and if it was feminine (e.g "jente" (girl)) you could use "ei" (but "en" is often also accepted).
If you're not familiar with grammatical genders, then this is just something you have to practice and after a while it will come naturally. If not, then anyone in Norway are going to understand you fine, it will just sound a little funny sometimes when you use the wrong article.
Maybe it's just me, but it seems strange that child is neuter, but dog is masculine. (Actually, more dogs are likely to be neutered than children. ;) ) But both are just as likely to be male or female. I guess it's not a case of making sense - just learning.