难怪 (không có gì ngạc nhiên)

A reason makes an absence understandable

• A friend did not attend. You learn that he was ill: no wonder难怪nán guàihe他tādid not come没来méi lái，it turns out / originally (context)原来yuán láiillness病bìng(completed / change)了le。 — No wonder he did not come; it turns out he was ill.
• no wonder难怪nán guài expresses the speaker’s understanding of the absence. It does not itself rule out every other reason for staying away.

React to an observation with no wonder难怪nán guài

• Use no wonder难怪nán guài + observation. The reason may appear nearby: it turns out / originally (context)原来yuán láihe他tāillness病bìng(completed / change)了le，no wonder难怪nán guàinot / haven't没méicome来lái。 reverses the order of the two ideas.
• The original not不bùcome来lái example can describe not coming in its context. For this completed absence, not / haven't没méicome来lái makes the event’s non-occurrence explicit.

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

Name the observation and its explanation

• it turns out / originally (context)原来yuán láihe他tāto check考kǎo(completed / change)了le第一名dì yì míng，no wonder难怪nán guàiso much这么zhè mehappy高兴gāo xìng。 — He came first; no wonder he is so happy. Coming first explains the happiness to the speaker.
• This is a reaction of understanding, not an instruction to be happy or proof that winning is his only source of happiness.

A reaction does not establish the evidence

• Someone guesses the absent friend is ill and says no wonder难怪nán guàihe他tādid not come没来méi lái. The phrase does not turn that guess into a confirmed medical fact.
• Check whether the reason was known or only proposed. A confident reaction and reliable evidence are different things.

Choose a real question for missing information

• If you still do not know why he missed the meeting, he他tāwhy?为什么wèi shén menot / haven't没méicome来lái？ asks for a reason.
• no wonder难怪nán guàihe他tādid not come没来méi lái。 presents the absence as understandable given a reason. It does not simply ask the same question.

Separate the reaction from proof

Read the full reference: HSK 5 grammar.