This is the 2903rd most frequent Russian word.
внук
Grandson.
The word 'внук' is used as the subject, referring to someone's grandson.
Мой внук ходит в школу.
My grandson goes to school.
The word 'внуком' is the instrumental case form of 'внук', indicating the object of the pride.
Я очень горжусь своим внуком.
I am very proud of my grandson.
The word 'внуку' is in the dative case, showing to whom the gift was given.