3週2日目 太るわけだ
Development・time・result
接続：普通形＋わけだ
No wonder; means; reasonable conclusion
注意 / Note：わけだ is the logical conclusion; わけではない partial negative; わけがない strong negative.
比較 / Compare：Same song: 〜わけではない／〜わけがない. These models are often mistakenly chosen for each other — carefully read the 接続 and nuances above and then decide.
よく出る形 / Common form：普通形＋わけだ。Two ways to use: explain (どうりで〜わけだ) and paraphrase (つまり〜わけだ). Or go with どうりで・つまり.
毎日ケーキを食べれば、太るわけだ。
Eating cake every day will make you gain weight.
彼は日本に十年いた。日本語が上手なわけだ。
They've been in Japan for ten years, no wonder they're good at Japanese.
五年も住んでいたのか。道理でこの辺りに詳しいわけだ。
Lived here for five years. No wonder you know this area so well.