〜わけだ
No wonder; which means (an inevitable conclusion)machine translation
FormationPlain form + わけだ (な-adjective + な, noun + という)
〜わけだ expresses an inevitable conclusion drawn from a known fact. When you hear the reason and it suddenly clicks, you use わけだ with a "no wonder!" nuance: 彼は十年日本に住んでいた。だから日本語が上手なわけだ (he lived in Japan for ten years — no wonder his Japanese is good). わけだ is also used to restate the same fact in a different way, equivalent to "that is, in other words."
Connection: plain form + わけだ; な-adjectives add な (上手なわけだ), and nouns usually connect with という (学生というわけだ). In a sentence, わけだ often appears with reasoning connectors like だから, つまり, それで.
Distinguish it from はずだ, which you learned at N4: はずだ is an inference made when you do NOT yet know the result ("by rights it should be so"), while わけだ accepts a result that is ALREADY clear and explains it ("so that's why"). That's why わけだ always needs a basis that has been stated or that both parties can see.
Examples
彼は十年日本に住んでいた。だから日本語が上手なわけだ。
He lived in Japan for ten years. No wonder his Japanese is so good.
電気がついていない。つまり、誰もいないわけだ。
The lights are off. Which means nobody's home.
今日は日曜日だから、銀行が閉まっているわけだ。
Today is Sunday — no wonder the bank is closed.