Form of Adverb

Question

How do we know which form of adverb has to be used...the one ending in '...ment' or the regular form?

- Marie chante le plus fortement. V/S Marie chante le plus fort.

- Jeanne parle le plus doucement. VS Jeanne parle le plus doux.

- Il conduit le plus lentement. V/S Il conduit le plus lent.

How do we know which form of adverb to use in such situations?

Accepted Answer

Bonjour Aashpreet,
using -ment is the regular way of forming adverbs in French. "Fort" is an exception here as its adjectival form can also be used as an adverb.
When an adjective, "fort" means loud or strong but when used as an adverb it means loudly whereas the adverb "fortement" means strongly.
Marie chante le plus fortement.  V/S   Marie chante le plus fort.
Jeanne parle le plus doucement.   VS Jeanne parle est le la plus doux douce.
Il conduit le plus lentement.  V/S  Il conduit est le plus lent. 
Take a look here: Le/la/les plus and Le/la/les moins = The most and the least (Superlative with Adjectives in French)
I hope this is helpful.
Bonne journée !