Learn Spanish (Latin America): El supervisor manda que escribas el reporte. - The supervisor orders you to write the report.

We use the subjunctive because mandar (to order) is a verb of influence. When one person (the supervisor) tries to influence or command what another person (you) does, the second verb must be in the subjunctive. The word que connects these two different clauses.

I thought mandar meant "to send". Why does it mean "to order" here?

Mandar is a flexible verb! It does mean "to send" (like sending a package or an email), but it also means "to order" or "to command" (think of the English word mandate). When used with que and the subjunctive, it almost always means "to order".

How do we know the supervisor is ordering me specifically, since there is no te before manda?

We know the order is directed at you because the second verb, escribas, is conjugated for tú (you). The sentence translates literally to: The supervisor orders that you write the report.

Can I use an infinitive here instead of que and the subjunctive?

Yes! With verbs of influence like mandar, you can sometimes drop que and use an indirect object pronoun plus an infinitive. You could say El supervisor te manda escribir el reporte. Both ways are very common and mean the exact same thing.