～ないことはない

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The ～ないことはない pattern is used to express "It's not that X isn't Y", using a double negative.

ことはない can be used in combination with negative verbs, い-adjectives, as well as な-adjectives followed by じゃない or ではない.

E.g. 食たべれないことはないと思おもうけど、やめたほうがいい。 "I don't think it's that you can't eat it, but you should stop (eating it)."

The ないこともない pattern is also possible and sounds less certain than its は variant.

E.g. 近ちかくないこともないが、歩あるいて行いくのは大変たいへんだ。 "It's not that it isn't close, but it's tough to walk there."

なくはない・なくもない are also possible and are more common in casual, spoken scenarios.

E.g. できなくはないけど、やりたくない。 "It's not that I can't do it, but I don't want to."

ことはなかった is also possible, and expresses "It wasn't that X wasn't Y", in the past tense.

E.g. あの映画えいがは、面白おもしろくないこともなかったけど、次つぎのは見みなくてもいいかな。 "It wasn't that that movie wasn't interesting, but I don't think I need to see the next one."

Summary

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  The ～ないことはない pattern is used to express "It's not that X isn't Y"

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  ことはない can be used in combination with negative verbs, い-adjectives, as well as な-adjectives followed by じゃない or ではない

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  The ないこともない pattern sounds less certain than its は variant

• 

  なくはない・なくもない are more common in casual, spoken scenarios