Learn French: Je ne mets jamais le chauffage avant l'hiver. - I never put on the heating before winter.

You use avant de when it is followed by an infinitive verb (an unconjugated verb), such as avant de manger (before eating). Because l'hiver (winter) is a noun and not a verb, you only need the preposition avant.

Does the verb mettre mean "to turn on" in this context?

Exactly! While mettre literally means "to put", it is very frequently used in French to talk about turning on systems, appliances, or screens. Saying mettre le chauffage is just like saying "to put the heating on" in English. You could also use the verb allumer (to turn on/to light).

Why do we need the article l' before hiver? The English translation is just "before winter".

In French, seasons (le printemps, l'été, l'automne, l'hiver) generally require a definite article, especially when talking about the season as a general concept. While English drops the "the" in phrases like "before winter" or "during summer", French keeps it: avant l'hiver.

Just as a refresher, why isn't there a pas in je ne mets jamais?