怪不得 (no wonder)

That explains it

• Your classmate speaks perfect English, and today you learn she grew up in London.
• no wonder怪不得guài bu deshe / her她tāEnglish (language)英语yīng yǔvariant of 说说shuō(degree marker)得deso这么zhè megood好hǎo，so it turns out原来yuán láishe / her她tāat / in在zàiLondon, capital of United…伦敦lún dūnto grow up长大zhǎng dà。 — "No wonder her English is so good — she grew up in London."

no wonder怪不得guài bu de + surprising fact (，so it turns out原来yuán lái + reason)

• no wonder怪不得guài bu de = "no wonder": something that seemed strange now makes sense.
• The reason often follows with so it turns out原来yuán lái, or comes before: so it turns out原来yuán láiit snowed下雪xià xuě(completed / change)了le，no wonder怪不得guài bu deso这么zhè mecold冷lěng.

so it turns out原来yuán láiit snowed下雪xià xuě了le，no wonder怪不得guài bu deso这么zhè mecold冷lěng。

Either order

• Fact first: no wonder怪不得guài bu dehe他tānot / haven't没méito come来lái，he他tāto fall ill生病shēng bìng(completed / change)了le。 — "No wonder he didn't come — he's ill."
• Reason first: he他tāyesterday昨天zuó tiānstayed up熬夜áo yè(completed / change)了le，no wonder怪不得guài bu detoday今天jīn tiānso这么zhè metired累lèi。 — "He stayed up late last night; no wonder he's so tired today."

he他tāyesterday昨天zuó tiānstayed up熬夜áo yè了le，no wonder怪不得guài bu detoday今天jīn tiānso这么zhè metired累lèi。

It needs a reason

no wonder怪不得guài bu de versus (it's) no wonder (that...)难怪nán guài

• (it's) no wonder (that...)难怪nán guài means the same and is very common: (it's) no wonder (that...)难怪nán guàiso这么zhè mecold冷lěng.
• no wonder怪不得guài bu de can also mean "can't blame": 这件zhè jiànmatter事shìno wonder怪不得guài bu dehe他tā — "You can't blame him for this."

this这件事zhè jiàn shìcan't blame怪不得guài bu dehim他tā。

Recap

Read the full reference: HSK 5 grammar — Complex sentence patterns.