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can someone please explain となる ? I read it everywhere but still don't know what it exactly means.
Here's just an example sentence:
I'm not fluent with Japanese and can't read Kanji, but I think となる means "to become" or "to change" into a final state.
This site might help give you a better explanation - It focuses on the differences of になる and となる, but also gives it's meaning.
この世界が心つなぐ鍵となる - I'm sorry I can't translate this but in Romaji it would look like this > kono sekai kokoro tsunagu kagi tonaru. Translated literally it might look like this > (kono)This (sekai)world (kokoro)heart (tsunagu)to tie, fasten or connect (kagi)key (tonaru)to change into final state.
So possibly > This world's heart is tied (or bound) to change?
Sorry can't read Japanese yet so hopefully this helps.