Learn Spanish (Latin America): Quiero azúcar en la taza, pero no me lo sirves. - I want sugar in the cup, but you are not serving it to me.

When you have two object pronouns together, the indirect object (who receives the action, like me) always comes before the direct object (the thing itself, like lo). A helpful trick is to remember people before things.

Why do me lo go before the verb sirves?

In Spanish, object pronouns must be placed directly before a conjugated verb. Since sirves is conjugated, me lo comes right before it. Notice that the negative word no goes before the pronouns, creating the order: no + pronouns + conjugated verb.

What does the lo replace here?

It replaces azúcar (sugar). azúcar is the direct object, which is the thing being served. Because it is treated as a masculine noun in this sentence, we use the masculine pronoun lo.

If the verb is servir, why is it spelled sirves here?

servir is a stem-changing verb. In the present tense, the e in the stem changes to an i for most forms, including the tú (you) form. That is why it becomes sirves.