核心问题
They came earlier than expected.
词典解答
语言学家们认为,这里省略了形式主语 it 和谓语动词 was 以及充当真实主语的动词不定式:
They came earlier than expected. = They came earlier than it was expected for them to come. You arrived earlier than necessary. = You arrived earlier than it was necessary for you to arrive. 类例：
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The girls swam faster than was expected. (CGEL) 这些姑娘们游得比预料的快。
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... the result is better than could be expected (CGEL) ……结果比可能预料的要好 (参见 than )