Learn Italian: Lui sa tutto, ma io non so niente. - He knows everything, but I know nothing.

They are both present tense forms of the irregular verb sapere (to know). Io so means "I know", and lui sa means "he knows". Because sapere is highly irregular, its forms don't follow the normal -ere verb rules and simply need to be memorized.

Why is there a non in front of so niente? Doesn't that make a double negative?

Yes, it does! In Italian, double negatives are grammatically correct and often required. When a negative word like niente (nothing) comes after the verb, you must put non (not) before the verb. Literally, you are saying "I do not know nothing."

I learned that Italian usually drops words like "I" (io) and "he" (lui). Why are they included in this sentence?

You're right that subject pronouns are usually dropped because the verb ending already tells you who is doing the action. However, Italians keep them when they want to emphasize a contrast. By keeping lui and io, the sentence strongly highlights the opposite situations: what he knows versus what I know.

Could I use tutti instead of tutto in this sentence?