Learn French: Il freine le plus prudemment pour sauver les pneus. - He brakes the most cautiously to save the tires.

It never changes! Because prudemment (cautiously) is an adverb, it describes the verb freine (brakes) rather than a noun. Adverbs are invariable, meaning they don't have a gender or number. Therefore, to make an adverb superlative, you always use the invariable article le followed by plus or moins.

How is the adverb prudemment formed, and why does it have a double M?

It is formed from the adjective prudent (cautious). When a French adjective ends in -ent, you create its adverb form by replacing -ent with -emment. This double-M ending is pronounced like 'ah-mahn'.

Why is the verb sauver (to save) left in the infinitive here?

It directly follows the preposition pour. Whenever you want to express purpose in French (meaning 'in order to' do something), you use pour followed by an unconjugated verb (the infinitive).

Doesn't the plural of words ending in -eu usually end in -x? Why is it les pneus?