Learn Turkish: Kıvılcım aniden alevlendi ve otomatik cihaz kendi kendine kapandı. - The spark suddenly flared up and the automatic device turned off by itself.

This is a reduplicated reflexive adverb, which is the focus of your current lesson. By placing the bare form kendi next to the dative form kendine (literally "itself to itself"), it emphasizes that the action happened entirely "by itself" without any outside interference. Here, it acts as an adverb telling us exactly how the automatic device shut down.

Does kapandı mean "was closed" (passive) or "closed itself" (reflexive) here?

The verb kapanmak can be either passive ("was closed by someone") or reflexive ("closed itself / shut down"). While context often tells you which one it is, in this sentence, the phrase kendi kendine completely removes any ambiguity. It makes it 100% clear that the verb is functioning reflexively: the device turned itself off.

How is the verb alevlendi built from the noun alev (flame)?

It uses the suffix -len (or -lan), which turns a noun into an intransitive verb, usually meaning "to acquire [noun]" or "to become [noun]". Here, alev (flame) + -len + -di (past tense) literally means "it acquired flames" or, more naturally, "it flared up".

Why does "suddenly" translate to aniden instead of just ani?