Learn Turkish: Dünkü olay bir rüya değildi. - Yesterday's event was not a dream.

The suffix -ki usually never changes, no matter the vowels in the word before it (like in yarınki or sabahki). However, dün (yesterday) and gün (day) are two of the very few exceptions where it follows vowel harmony and becomes -kü. So, "yesterday's" is always dünkü.

Why can't I just say dün olay to mean "yesterday's event"?

In Turkish, dün tells you when an action happened (like "I went yesterday"). To use a time word to describe a noun directly (like "yesterday's event" or "tomorrow's exam"), you have to turn it into an adjective. Adding the -ki suffix does exactly that!

How is the word değildi formed?

It is the negation word değil (not) combined with the past tense ending -di (was). Together, değildi means "was not". It is the standard way to say that a noun or adjective "was not" something in the past.

Why is bir placed before rüya?

Bir means "a" or "an". Just like in English, when you are saying what a specific thing is or isn't, you use bir. Saying bir rüya değildi makes it clear that the event was not "a dream".