Learn Turkish: Cehennem kelimesinin nereden geldiğini öğrendik. - We learned where the word 'hell' comes from.

In Turkish, indirect questions are built by turning the verb into a noun clause using -dIğI (for past/present) or -(y)AcAğI (for future). Here, the verb "comes" becomes geldiği (literally "its coming"). When you place the question word nereden (from where) in front of it, you get nereden geldiği (where it comes from).

Why is there a genitive suffix (-nin) on kelimesinin?

Because it is the subject of the embedded -dIğI clause! In Turkish, the subject of a -dIğI or -(y)AcAğI clause must take the genitive case (acting like a possessor). The subject here is Cehennem kelimesi (the word 'hell'), so it gets the genitive -nin to link it to the action: literally, 'the word hell's coming'.

Why does geldiğini end with -ni instead of just being geldiği?

The base word here is geldiği (its coming). The final -ni is the buffer letter -n- plus the accusative case suffix -i. The entire clause "where the word hell comes from" is the direct object of the main verb öğrendik (we learned). Since we learned a specific, defined thing, the whole noun clause needs the accusative case.

Why is it Cehennem kelimesi instead of just Cehennem kelime?