"You don't have to worry."
Translation:Ustedes no tienen que preocuparse.
You don't know. If could be singular as well. Next time take a look at your answer to be sure that your subject and verb agree. For example if you used "Tu" then you need tienes if you used Ustedes you need tienen. And of course this one is harder because it introduces the reflexive which we don't use as much. Here you would need te for the tu and se for ustedes. As you can see in this thread, other pronouns could also be used but everything has to match within the sentence.
It's not conjugated, you add the se at the end to aim it at ustedes/you all.
You'll learn more about this later too. I believe you can also say "se tienes que preocupar" and get the same meaning. It's just that the se (or te, me, nos...) has to go either before the first verb or at the end of the last verb.