英语主语从句求教：1：他们碰巧出席了2：我不了解的是,他为什么改变主意了3：他对我所说的话让我迷惑

很高兴为你作答
1.It happened that he was present.一般来说主语从句都是放在句首.但有时为了使句子保持平衡,避免”头重脚轻”,常用it作形式主语,从而把主语从句放在后面.例如It is known
all /said/told/heard/ reported/decided/ suggested/advised/ ordered/thought/considered hoped… that.等等
2.what i did not understand is why he changes his mind
3.what he said to me makes me confused.
2和3 都可以用what引导的主语从句来表达
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They happened that they attended the meeting.
i don't understand What I reason why they changed their mind.
What he said to me makes me confused,
祝你成功

It happened that they attended the meeting.
What I do not understand is the reason why they changed their mind.
What he said to me makes me confused,