Learn French: Le projet est un succès, je le garantis. - The project is a success, I guarantee it.

It acts as a neuter pronoun replacing the entire previous idea: Le projet est un succès (The project is a success). Instead of repeating the whole phrase by saying je garantis que le projet est un succès, you just use le to say "I guarantee it."

Would le change to la if the first part of the sentence was about a feminine noun, like La réunion est un succès?

No, it stays le. Because it is replacing a complete clause or idea rather than a specific noun, it is considered "neuter." This neuter pronoun is always le, regardless of the gender of any nouns mentioned in the idea it replaces.

What kind of verb is garantis?

It comes from garantir (to guarantee), which is a regular -ir verb. Just like finir (to finish) or choisir (to choose), it follows the standard pattern where the plural forms get an "iss" sound (for example, nous garantissons).

Why does succès have an s at the end even though it is singular (un succès)?