难怪 (no wonder)

The mystery solves itself

• (it's) no wonder (that...)难怪nán guài = "no wonder / that explains it".
• (it's) no wonder (that...)难怪nán guàihe / him他tā不来bù lái，original原来yuán láiillness病bìng(completed / change)了le = "no wonder he didn't come — turns out he's ill."
• It reacts to a fact that suddenly makes everything click.

(it's) no wonder (that...)难怪nán guài + Observation (，original原来yuán lái …)

• (it's) no wonder (that...)难怪nán guài says "that's why" once the reason appears.

no wonder难怪nán guàihe他tānot不bùcome来lái。

• (it's) no wonder (that...)难怪nán guàihe / him他tā不来bù lái = "no wonder he's not coming."

Recap

Read the full reference: HSK 5 grammar — Verbs.