出门前，他检查了所有的煤气和灯是否都关了。
Before leaving the house, he checked that the gas and all the lights were off.
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before英 /bɪˈfɔː/|美 /bɪˈfɔr/
leaving英 /ˈliːvɪŋ/|美 /ˈlivɪŋ/
解释
`(Before) (leaving)` 是 before + -ing 构成的非限定从属分句，在句首表示 leaving the house 发生在 checked 之后，即先 leaving 后 checked。隐含主语与主句主语 he 一致。
Before leaving the house, he checked that the gas and all the lights were off.
`(Before) (leaving)` 是 before + -ing 构成的非限定从属分句，在句首表示 leaving the house 发生在 checked 之后，即先 leaving 后 checked。隐含主语与主句主语 he 一致。