"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"
"Tener + que + infinitive" is the way to say "Have + to + infinitive" in Spanish, "No tienes ser" sounds weird, like "You don't have be".
As a Spanish learner I can imagine times when estar is appropriate for this translation e.g. You don’t have to be so formal (with my friends only with my co-workers.) I’m not referring to how one should dress, but rather how one would act. Would a native Spanish speaker never use estar? It wasn’t accepted by Duo in my response: No tienes que estar tan formal.