Learn Turkish: Tutup kabahatini arsızca inkâr etti. - He went and shamelessly denied his fault.

Literally, yes. But here it is acting as a converb of audacious or unexpected action, similar to saying "he went and..." or "he took it upon himself to..." in English. When paired with a main verb like inkâr etti, tutup emphasizes that the action was surprisingly bold, inappropriate, or completely out of line.

Let's break down the suffixes on kabahatini. Why are there two -i endings?

The first -i is the third-person possessive suffix (his fault). The second -i is the definite accusative case marker, used because this specific fault is the direct object of the verb inkâr etti. The -n- in between is the standard buffer letter used whenever you add a case marker to a third-person possessive suffix.

What exactly does the -ca suffix do in arsızca?

It turns the adjective arsız (shameless or impudent) into an adverb of manner, meaning "shamelessly". You will frequently see -ca or -ce attached to adjectives that describe human character or behavior to explain exactly how an action is being performed.

Could I use suçunu instead of kabahatini here?