DidiMihova

I know that many years past since this was posted, but in case there is someone wondering the mistake is that 余計 is a な形容詞 na-adjective and you need a noun to connect it with 余計なこと言わないで。 In 余計があったら、ひとつ貸してもらえませんか。余計 has been used as noun which it is not. But I also don't think 余計 is a good choice at all. Maybe the sentence would be better as 使わないなら、ひとつ貸してもらえませんか。

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