Learn Spanish (Spain): Si haces una reforma, se necesitan albañiles. - If you do a renovation, builders are needed.

This is the pasiva refleja (passive with se), which you are learning in this lesson. Even though it translates to 'builders are needed', grammatically, albañiles (builders) is the subject of the sentence. Because albañiles is plural, the verb must also agree and be plural (necesitan).

Why does the sentence use haces (you do) but then switch to se necesitan (are needed)?

The first half uses the informal tú form (haces) to mean 'you' in a general sense. The second half shifts to the passive voice (se necesitan) to focus on what is required, without saying exactly who needs them. Spanish speakers frequently mix these two ways of making general statements in the same sentence.

Does reforma mean 'reform' in Spanish?

It can in political or social contexts, but here it is a false friend. In the context of houses and construction, una reforma means a 'renovation' or 'remodeling'. If you say you are doing a reforma at home, Spanish speakers will immediately know you are having building work done.

Why do we use the present tense haces after si instead of the subjunctive?

When talking about highly likely or real possibilities (like needing builders for a renovation), si (if) is always followed by the regular present tense. You never use the present subjunctive directly after si in Spanish.