New known word?

How do you do when a word is derived? I mean, when you know the meaning of all the derivatives you mark them all as known or only the main word?
ex: know, known, knew
Do you mark only “know” as know and delete the others two or mark the 3 as known?
It gives the sensation that we know more words that we actually do, dont you think?

The goal of the known words counter is to show your progress. It never shows the real number of the words you know. I guess it’s impossible to count every words you know because some words will be forgotten in 3 months, other words will be forgotten in a year but in this time you will learn some other new words.
This counter shows the forms of the words you know.
So, relax and mark know, known and knew 3 times.

Thank you Ress