This is the 6003rd most frequent Turkish word.
verirken
"While giving" or "when giving."
In this sentence, 'verirken' is used to describe an ongoing action (giving) at the time when the emotion (happiness) was felt. It is derived from the verb 'vermek' (to give) combined with the suffix '-irken' which conveys simultaneous actions.
Elmayı arkadaşıma verirken çok mutlu hissettim.
I felt very happy while giving the apple to my friend.
Here, 'yerleştirirken' shows the simultaneity of the actions (placing and dropping), similar to 'verirken', but applied to the verb 'yerleştirmek' (to place).
Kitapları rafa yerleştirirken birini yere düşürdüm.
I dropped one of the books while placing the others on the shelf.
Although not 'verirken', the structure is similar; 'etmek üzereyken' shows an action just about to begin. This serves as a comparative structure to 'verirken', which emphasizes ongoing simultaneous actions.