Doğum günün kutlu olsun

why it is not "Doğn"?

Thanks,

doğum is like bilim or ölüm, it is a noun made of a verb root.

                öl|mek ´to die´ > öl|üm ´death´

                bil|mek ´to know´ > bil|im ´knowledge´

                doğ|mak ´to be born´ > doğ|um ´birth´

-Im is not a 1st person possessive suffix in doğum but a derivational suffix. So it doesn´t chance.

2nd person sg shows in gün|ün ´your day´.

Ahhh... i saw a thread before where you were explaining the -im suffix that is used to change verb to noun but i didn´t get it well at that time.

so to say happy birthday to him i should say "günü",

and to you (plural)"gününüz"

Evet HaNNo.

(Just remember making deverbal nouns doesn´t work with every verb this way. The use of derivational suffixes is limited and unpredictable.)

And in the cases above olsun also has to be changed into ol and olsunuz?

No. The wish always concerns 3rd person sg (gün).

So literally the phrase is

                        May (your, his) birthday be happy.

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