本当よ響ちゃん。私たちは帰らなかったんじゃなくて帰れなかったの
響 - Hibiki (name of a character)
So the storie goes like Hibiki goes in search of her friends who haven't come back in 2 weeks. When she founds them she asks them "why havent you come back" And they respond with the above asked
So how do they differ ?
[帰]{かえ}れる is the potential form of 帰る. So this means "It's true, Hibiki. It's not that we just didn't come [or go] back, it's that we couldn't come back". Presumably the speaker is explaining to Hibiki that they and their companion[s] were prevented from coming home by force majeure - they didn't just decide not to of their own volition. The general rule for making the potential form of verbs like かえる is: replace the final syllable of the dictionary form with the syllable from the same column of the kana chart that has the vowel e and then add ru. Thus: いく/いける、よむ/よめる、はなす/はなせる、しぬ/しねる、はなす/はなせる、もつ/もてる and so on. This form then inflects like たべる .
