Learn Turkish: Dün yazdığın yorumu kimse okumadı. - Nobody read the review that you wrote yesterday.

This is the object relative participle in action! We take the root verb yaz (write), add the suffix -dık (which forms a participle meaning "that which is done"), and then add the possessive ending -ın (your). It acts like an adjective describing the review, literally meaning "your written" or "that you wrote".

Why is there a soft g (ğ) in yazdığın instead of a k?

The base suffix is -dık. However, when we add the possessive suffix -ın (your) right after it, the hard k is followed by a vowel. To make it easier to say, the k softens into a ğ. This is a very common consonant mutation in Turkish.

Why is there a -u at the end of yorumu?

That is the accusative case marker. It points out the specific direct object receiving the action of the verb okumadı (did not read). Since we are talking about a specific review (the exact one you wrote yesterday), we must add the accusative -u to yorum (review).

If kimse means "nobody", why is the verb okumadı negative? Doesn't that mean "nobody didn't read"?