Learn Spanish (Latin America): El director le manda leer la partitura del violín. - The director orders him to read the violin sheet music.

Both are actually correct! With verbs of influence like mandar (to order), permitir (to allow), and prohibir (to forbid), you can use an indirect object pronoun plus the infinitive, or you can use que plus the subjunctive. El director le manda leer is just a slightly more concise alternative to El director le manda que lea.

Why is the pronoun le used instead of the direct object lo?

In this structure, the person receiving the order is the indirect object, which is why we use le. You can think of it like this: the direct object (the thing being ordered) is the action itself (leer la partitura), and the director is ordering that action to him.

If I decide to use the subjunctive instead, where does the pronoun le go?

It stays right where it is, before the verb of influence! The sentence would become El director le manda que lea la partitura del violín. The le still indicates who is receiving the order from the director, while the conjugated verb lea shows who is performing the action of reading.

Why does the sentence say del violín rather than just de violín?