怪不得 / 难怪: no wonder
【高36】【高39】语气副词 / adverbs
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
怪不得 and 难怪 both say "no wonder": a fact that was puzzling becomes logical because you know (or have just found) its cause. Two orders are possible: the reason first, then 难怪 / 怪不得 + the fact (这道题非常难，难怪她一时答不出来); or the fact first, then the reason revealed by 原来 (怪不得她没去爬山，原来昨天下雨了).
The two words are almost interchangeable; 怪不得 is a little more spoken. It is always an observation: you understand, you are not surprised (for surprise, use 没想到 or 竟然), and it is never a question. It has nothing to do with the rhetorical question 难道, even though 难怪 and 难道 share a character.
Both can also work as a verb meaning "you can't blame": 这事怪不得你.
Examples
这道题非常难，难怪她一时答不出来。
Zhè dào tí fēicháng nán, nánguài tā yìshí dá bu chūlái.This question is very hard, so no wonder she can't answer right away.
怪不得她没去爬山，原来昨天下雨了。
Guàibude tā méi qù páshān, yuánlái zuótiān xià yǔ le.No wonder she didn't go hiking: it rained yesterday.
他是东北人，难怪他不怕冷。
Tā shì Dōngběirén, nánguài tā bú pà lěng.He's from the Northeast, so no wonder he isn't afraid of the cold.
你天天熬夜，怪不得没精神。
Nǐ tiāntiān áoyè, guàibude méi jīngshen.You stay up late every day, so no wonder you're listless.
怪不得今天路上这么堵，原来前面在修路。
Guàibude jīntiān lùshang zhème dǔ, yuánlái qiánmiàn zài xiū lù.No wonder the road is so jammed today: they're repairing it up ahead.
她在法国住了十年，难怪法语说得那么地道。
Tā zài Fǎguó zhù le shí nián, nánguài Fǎyǔ shuō de nàme dìdao.She lived in France for ten years, so no wonder her French sounds so authentic.
Common mistakes
✗ 怪不得他来了！ → ✓ 没想到他来了！ (怪不得 explains a fact by its cause; for a surprise, say 没想到 or 竟然).
✗ 难怪他不怕冷吗？ → ✓ 难怪他不怕冷。 (it is a statement, never a question; the rhetorical question is 难道……吗).
✗ 他不怕冷，难怪他是东北人。 → ✓ 他是东北人，难怪他不怕冷。 (难怪 introduces the fact being explained; the cause comes first, or after 原来).