Learn Hardcore Spanish (Spain): Cuando hace frío, la caldera no funciona bien. - When it is cold, the boiler does not work well.

Breakdown of Cuando hace frío, la caldera no funciona bien.

Why is it hace frío and not something like es frío or hay frío?

In Spanish, weather expressions often use hacer:

This is just the normal Spanish pattern. The hace here does not mean makes in a literal sense.

es frío usually describes something as being cold by nature or characteristic, for example El clima aquí es frío.
hay frío is not the normal way to talk about the weather.

So in this sentence, Cuando hace frío is the natural way to say when it’s cold.

What is the subject of hace here?

There is no real subject in the English sense. This is an impersonal weather expression.

In hace frío, the verb is always third person singular:

You do not normally ask what makes cold? The phrase works as a fixed structure.

Why is it cuando + present indicative instead of subjunctive?