Learn French: Soyez élégant et ponctuel pour votre rendez-vous.

Soyez is the command form (imperative) of être (to be) for the vous form. Être is an irregular verb, so its command forms (sois, soyons, soyez) are completely different from its normal present tense forms.

Since soyez is the "vous" form, shouldn't élégant and ponctuel be plural?

Not necessarily! In French, vous is used for a plural "you" but also as a polite, formal "you" for a single person. Because the adjectives élégant and ponctuel are singular (they have no -s at the end), we know this sentence is speaking politely to one man.

How would this sentence change if we were speaking to a woman?

You would need to make the adjectives feminine. The sentence would become: Soyez élégante et ponctuelle pour votre rendez-vous.

Does rendez-vous always mean a romantic date?

Not always! While it means "date" in a romantic context, un rendez-vous is also the standard word for any appointment or meeting, like going to the dentist or meeting a client.