How do you say I understand EVERYTHING in French?

There are a few ways to say "I understand everything" in French, depending on the nuance you want to convey:

Formal:

* Je comprends tout. (This is the most literal and common way to say it.)

* Je comprends parfaitement. (This emphasizes that you understand completely.)

Informal:

* J'ai tout compris. (This is more casual and direct.)

* Je sais tout. (This implies that you know everything, not just understand.)

More emphatic:

* Je comprends absolument tout. (This strongly emphasizes complete understanding.)

* Je comprends tout à fait. (This is similar to "perfectly" in English.)

The best choice will depend on the context and your relationship with the person you're speaking to.