わけだ— That Explains It

• 彼は10年間日本に住んでいた。道理で日本語が上手なわけだ。

  Kare wa juunenkan Nihon ni sunde ita. Douri de Nihongo ga jouzu na wake da.

  He lived in Japan for ten years. No wonder his Japanese is good.

  上手な + わけだ presents the fluency as the natural result of living in Japan.

• エアコンが壊れている。だから暑いわけだ。

  Eakon ga kowarete iru. Dakara atsui wake da.

  The AC is broken. That's why it's hot.

  暑い + わけだ draws the logical conclusion from the broken AC.

• 全員が反対した。つまり、この案は通らないわけだ。

  Zen'in ga hantai shita. Tsumari, kono an wa tooranai wake da.

  Everyone opposed it. In other words, this proposal won't pass.

  通らない + わけだ restates the situation as a reasoned conclusion.

• ・Often paired with 道理で, だから, or つまり to signal the deduction.
• ・Expresses a conclusion the speaker arrives at, not a personal intention.
• ・Register: neutral; common in both speech and writing.

Commonly confused with

The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones わけだ is tested against.

• ～あまり(に) — So Much That It Overwhelms
• ～おかげで — Thanks To
• ～せいで — Because Of (And I Blame It)
• ～せいか — Perhaps Because Of
• ～からといって — Just Because
• ～ことから — Judging From The Fact
• 次第だ／次第で — It All Depends
• だけあって — Living Up to It