Learn Turkish: Hakkında konuştuğumuz editör bu kapağı beğendi. - The editor about whom we talked liked this cover.

In English, we use a relative pronoun like 'about whom we talked.' Turkish handles this by putting a postposition with a 3rd-person possessive right before the relative clause. Hakkında literally means 'about him/her/it.' Combined with the -dIk participle, the phrase translates word-for-word as 'about-him/her our-talked editor', linking the postposition back to the head noun.

Could we say onun hakkında konuştuğumuz editör to make the 'about him' part clearer?

You technically could, but native speakers almost always drop onun (his/her) here. The 3rd-person possessive suffix already embedded inside hakkında does the job of pointing to the editör, so adding onun is unnecessary and can sound clunky.

Why does the word kapak change to kapağı in this sentence?

The verb beğendi (liked) acts on a direct object. Because the cover is specific, pointed out by the word bu (this), it requires the accusative case suffix -ı. When you add a vowel suffix to a word ending in k, the k softens to a ğ, making it kapağı.

Why use beğendi instead of sevdi for 'liked'?