Learn Turkish: Yarınki umut büyük. - Tomorrow's hope is big.

The suffix -ki attaches directly to time words like yarın (tomorrow) to turn them into adjectives. Here, yarın + -ki becomes yarınki, which acts as an adjective describing the noun umut (hope). It translates to "tomorrow's" or "the one belonging to tomorrow".

Wait, yarın has back vowels (a, ı). Why didn't vowel harmony change the suffix to -kı?

The suffix -ki is a famous exception to Turkish vowel harmony rules! It almost always stays as -ki, regardless of the vowels in the root word. The only common exceptions you will see are dünkü (yesterday's) and bugünkü (today's), which round out to match the 'ü' or 'u'.

Could I say yarının umudu instead of yarınki umut to mean "tomorrow's hope"?

Yes, you absolutely could. Yarının umudu uses the genitive-possessive structure (literally "the hope of tomorrow"). However, using -ki is a shorter, very common alternative that neatly turns the time word into a simple adjective.

Where is the word for "is" in this sentence?