わけではない vs わけがない vs わけにはいかない

It is not the case that; not always so

わけではない denies a conclusion or weakens a claim to mean it is not the case that something holds. It is often used to correct a misunderstanding or to show that something is not always true, as in 高い店がおいしいわけではない. It follows the plain form, such as する, しない, した, i-adjectives like 高い, na-adjectives with な like 静かな, and nouns with である or だ, as in 学生であるわけではない. The tone is explanatory and neutral, and care is needed to keep the negation on わけ rather than making the predicate itself fully negative.

1. そういう類たぐいのものは、どこにでもあるわけではない。

   That kind of thing can't be found just anywhere.

2. しかしこれは昔から常にそうであったわけではない。

   However, this has not always been true.

There's no way; impossible; can't be true

わけがない expresses strong conviction that something is impossible or cannot be true, like saying there is no way it could happen. It follows the plain form of verbs and i-adjectives, and な after na-adjectives and nouns, as in 知っているわけがない or 内向的なわけがない. である may replace な with nouns in more formal writing. It is emphatic and often sounds argumentative, so it suits firm denials among people you know well.

1. カナダ人じんならそんなこと言いうわけがない。

   No Canadian would ever say that.

2. そんな映画見るわけがない！

   There's no way I'd watch that kind of movie.

unable to do; not permitted to do

わけにはいかない expresses that someone cannot do something because of duty, social expectations, or outside circumstances, even if they want to. It attaches to the dictionary form of a verb for not doing something, as in 行くわけにはいかない, and to the ない form for not avoiding something, as in 行かないわけにはいかない. It describes a social or situational restriction rather than physical inability, and it is often used to explain the speaker's own constrained choice.

1. 君きみがそうすることを許ゆるすわけにはいかない。

   I can't allow you to do that.

2. 何事も思い通りにするわけにはいかない。

   You cannot have your way in everything.