Learn Turkish: O günün bana zehrolduğunu hissettim. - I felt that that day was ruined for me.

The noun zehir (poison) combines with the auxiliary verb olmak (to become/be). When this happens, the narrow vowel 'i' in the second syllable of zehir drops. This is called vowel elision. In Turkish, whenever an auxiliary verb compound undergoes a sound change like losing a vowel, the rules dictate that the two words must be written together as a single word: zehrolmak.

Why are there two "s"s in hissettim, and why is it also a single word?

This is another compound verb, made from his (feeling) and etmek (to do). When the Arabic loanword his is attached to an auxiliary verb starting with a vowel, its final consonant doubles. This is called consonant gemination. Just like with zehrolmak, because a sound change occurred (his becoming hiss-), orthographic rules require the compound to be written as one word: hissetmek.

Why does the sentence use the genitive o günün instead of just the nominative o gün?

The main verb hissettim (I felt) is taking an entire subordinate clause as its object ('that that day was ruined'). In Turkish, this 'that' clause is built using the participle -dık (seen inside zehrolduğunu). A strict rule of these participle clauses is that their subject must take the genitive case. Therefore, the subject o gün becomes o günün.