わけだ says that something follows from what you have just learnt: "no
wonder", "that's why". It follows the plain form of a verb or い-adjective;
a な-adjective or noun takes な. わけです is the polite form.
。。
He lived in Japan for ten years? No wonder his Japanese is good.
、。
Today is a holiday, so that's why the bank is closed.
さん3、。
I only slept three hours, so of course I'm sleepy.
The fact itself was already known; わけだ adds that you now see why. Just
stating it – 眠い – has none of that.
道理で…わけだ
at the start means "no wonder" outright, and it is nearly
always closed with わけだ.
。。
No wonder it's cold. It's snowing.
。。
No wonder it's quiet. Everyone's gone out.
わけだ or はずだ
is an expectation before you check: "it should be". わけだ is the
understanding you reach after the fact.
、。
Today is a holiday, so the bank should be closed.
。。
Oh, today is a holiday. That's why the bank is closed.
Summing up what someone said
With and often というわけ, the speaker restates what the other
person has said in their own words and checks it: "so what you're saying
is…".
、。
So next week is a holiday, then?
、。
So he's not coming, then?
Lingaku