It is in sb to do sth. 做某事是某人的责任、份内的事。
例如:I never had to tell him about his schoolwork. It was in him to do it. 我从来不必督促他作功课。那是他份内的事。
whatever work it was in him to do 他职责范围内的所有该做的事。
He has achieved whatever work it was in him to do。他做完了所有他该做的事。
in him并不是不定式的主语。to do sth为真正主语,it为形式主语。in him为介词短语作表语。to do sth is in sb. 做某事是某人的职责。主系表结构。
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