фаҳмидан

Шумо ҳеҷ чизро намефаҳмед!
You don’t understand anything!
Агар бештар хонед, бештар мефаҳмед.
If you read more, you will understand more.

Bonus:
Note – хондан can also mean to study, so you could also translate the second sentence as “If you study more, you will understand more”.
Words related to фаҳмидан:
фаҳмиш: understanding, intelligence (noun)
фаҳмо: understandable, clear (adj.)