"He is a student but they are not students."
Translation:Is dalta é ach ní daltaí iad.
I don't understand where the verb "to be" is in the second half of the answer. I thought that "ní" by itself pretty much translated as "not" and therefore needed to be paired with a separate verb, while "níl" was a contraction of "ní bhfuil" so it has the accompanying verb included. What am I missing here?
There is no rule for vowels following each other (you've seen plenty of sentences that start with tá an). There are cases where the sounds of two vowels would blend into each other without some sort of buffer to keep them apart, and there are times when that blending isn't a problem, so you don't need a buffer.
In the case of pronouns like é, í and iad, you use a h- prefix after ní, cé and cá*.