Learn French: Tu dois envoyer un message au client. - You have to send a message to the client.

Dois is the "you" (tu) form of the verb devoir, which means "must" or "to have to". The present tense conjugation for tu is always tu dois.

Could I translate Tu dois as "You must" instead of "You have to"?

Yes! In French, devoir covers both "must" and "have to". Tu dois envoyer can mean either "You have to send" or "You must send", depending on what sounds more natural in English.

Why is envoyer (to send) left in its full dictionary form instead of being conjugated?

In French, when two verbs are used together to express a single idea, only the first one is conjugated. Since dois is already conjugated to match tu, the second verb (envoyer) stays in its dictionary form (called the infinitive).

Why does it say au client instead of à le client?

In French, the preposition à (to) and the masculine word le (the) always combine into the single word au. You will never see à le written side-by-side; they must always shrink into au.