Recently, I came across a question about the difference between “はずだ” and “わけだ”, which appear in JLPT N3 level vocabulary.
So I wrote a brief explanation about it shown below.
And, an example conversation is as follows:
Joe had a bag, passed it to Betty.
ジョー : 重く感じるはずだよ。 (1)
Joe: It must be heavy for you.
Betty: Yeah, it is… why?
Joe: Look, it’s the reason…
Joe opened the bag, and Betty saw many books in it.
Betty: Oh, I see why it’s so heavy!
(1) Betty might not feel it heavy if she were a powerful woman; Joe didn’t know how betty would feel, he could just guess it. Then, “重く感じる” is Joe’s guessing about Betty’s feeling.
In such a case, you can’t use “わけだ” there,
“はずだ” is adequate for guessing.
(2) Joe knew the reason of the heavy weight. It is not guessing. So he used “わけだ”.
(3) Betty just got to know the reason, In such a case, both of “はずだ/わけだ” are available.