Learn Dutch: Ik denk dat jij gelijk hebt. - I think that you are right.

Because the second part of the sentence starts with dat (that). The word dat is a subordinating conjunction. It introduces a dependent clause and forces the working verb (hebt) to move to the very end of that clause.

Why does the Dutch sentence use hebt (have) when the English translation says are?

In Dutch, the expression for 'to be right' is gelijk hebben (literally 'to have right'). Instead of saying you are right, Dutch speakers say you have right. That is why it uses hebt (have) matching the pronoun jij.

Could I use je instead of jij in this sentence?

Yes, you absolutely can. Using jij instead of je just adds a bit of emphasis. Here, it highlights that you are the one who is right, perhaps in contrast to someone else who might be wrong.

I have seen commas before dat in other sentences. Is a comma required here?

For short sentences like Ik denk dat jij gelijk hebt, a comma before dat is optional and usually left out. You will see commas used more often when the clauses are longer to make them easier to read.