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Noun + である/という+ わけ + (だ)"as you know" "the fact is that--"
Na-Adj. + な + わけ +(だ)
i-adj. + わけ +(だ)
Verb Inf. + わけ +(だ)

(1)
By adding わけだ at the sentence final position, the speaker emphasis's A fact or the truth which the hearer might not know.
In this construction, わけだ should be always affirmative.
Please don't confuse this usage of わけ with the usage introduced below

(1).私は子供の言語学習を研究しているわけですが、この研究からいろいろな面白いことがわかります。
(I have been doing research on child language learning , as you know, and various interesting facts have become known from this research.)
昔は、一般的に言って、結婚して初めて一人前と見なされていたわけです。
( Lit: Generally speaking, as you may know, in old days, the fact is that after having married, A man is for the first time regarded to be A full-fledged person.)
戦争は大きな犠牲 (=ぎせいsacrifice) を伴うわけで、我々はなんとしてもこれを避けなければならない。
(War is accompanied by great sacrifice, as you know, and we have to avoid it at any cost.) 避ける(さける=avoid)

Other important usage of わけ:
(1)
(Proposition X), (therefore Y) +わけだ
"Given X, one is logically led to the conclusion Y."
Proposition X : 田中さんは4年間ドイツの大学で勉強した。
Conclusion Y : ドイツ語が上手なわけだ
田中さんは4年間デュイスブルグ大学でドイツ語を勉強したのだから、ドイツ語が上手なわけだ。
(Mr.Tanaka studied German for four years at Duisburg University, it is natural that he is good at German.)
This usage of わけだ is interchangeable with 「はずだ」and is often accompanied with 道理で(どうりで= No wonder.....).
(2)
Sentence +(という) わけではない "it does not mean that...""It does not follow that..."
「わけではない」negates what one would generally conclude from previous statements or situation.
あまり日本語を話しませんが、日本語ができない(という)わけではありません。
(I don't speak Japanese much, but that does not mean that I can't speak Japanese.)
A:この病人はまったく助からないのですか。B:いえ、そういうわけではありません。
(A:Can't this patient be saved at all? B:No, it is not that he can't be saved.)
(3)
Sentence + わけにはいかない "can not do ---" "there is no choice but to--"
"out of the question"
「わけにはいかない」is used to express that even though one wants to do something,he cannot do so, due to external circumstances and psychological reasons originating from social norms, customs, common sense.
熱があっても、きょうは大事な会議があるから、会社を休むわけにはいかない。
(Even though I've got A terrible fever, I can not take the day off, because there is an Important meeting today.)
こんな高価なものをいただくわけにはいきません。(I have no reason to receive such an expensive gift.)
(4)
Sentence (non-past)+ わけがない "there is no possibility of --""There is no reason why --"
あなたには私の気持ちがわかるわけがないわ。(It is impossible for you to understand my feelings.)
あと人がそんなことを言うわけがないでしょう。(There is no reason for her to say such A thing.)
こんなにやさしい問題がわからないわけがない。
(It is impossible for you not to be able to solve such an easy question.)

This strong negative 「わけがない」is basically interchangeable with 「はずがない」,
Though there is A slight difference in its nuance.
e.g.
あの人に私の気持ちがわかるわけがない。
Lit:There is no possibility that he can understand my feelings.
あの人に私の気持ちがわかるはずがない。
Lit:It is absolutely impossible to expect that he can understand my feelings.
こんなにやさしい問題がわからないわけがない。
(It is impossible for you not to be able to solve such an easy question.)

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