Learn Spanish (Latin America): Me la vas a arreglar si la madeja se enreda. - You are going to fix it for me if the skein gets tangled.

When you have a conjugated verb followed by an infinitive (like vas a arreglar), you have two choices for pronoun placement. You can either put them before the conjugated verb (Me la vas a arreglar) or attach them directly to the end of the infinitive (Vas a arreglármela). Both are perfectly correct, but placing them before the conjugated verb is very common in everyday Spanish.

Since the tangling hasn't happened yet, why is enreda in the present indicative rather than the subjunctive?

In Spanish, si (if) is almost never followed by the present subjunctive. When talking about a possible or likely condition in the future, you use si plus the present indicative. Even though the skein hasn't tangled yet, si la madeja se enreda expresses a real possibility, so the present indicative is the correct choice.

What is the function of se in se enreda?

The verb enredar means "to tangle" (acting on something else). By using the pronominal form enredarse, it translates to "to get tangled" or "to become tangled." The se shows that the subject (la madeja) is undergoing the change of state itself, rather than tangling another object.

What does the la in Me la vas a arreglar refer to?