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The following sentence I saw confuses me:

わたしは すしが たべられないです。

So it takes the verb たべる (to eat), turns it into the potential form たべられる(to be able to eat) and then conjugates it in to the informal negative たべられない (cannot eat). Happy with all of that. But then it adds です onto then end, which I'm guessing is supposed to make it formal. My question is, why does it not just say:

わたしは すしが たべられません。

Are both forms correct? Can you always make a formal sentence by adding です to an informal conjugation and why would you choose one way over the other? Thanks.

marked as duplicate by virmaior, broccoli forest, Darius Jahandarie, cypher, oldergod Feb 9 '15 at 0:22

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