"You do not have to be so formal."
Translation:No tienes que ser tan formal.
This seems to be a good example of subtle differences between ser and estár. Is formal a personality characteristic or a temporary condition.
Yes, when you want to imply formal as a characteristic or manners, then you say "ser formal". However, if you want to use formal as in dressing, then you simply use the verb "vestir"
Why the que? I put "tú no tienes ser tan formal" and it was marked wrong.