In the Practical English Usage 3rd, it says:
We can leave out words after 'as' and 'than', if the meaning is clear.
The weather isn't as good as last year. (= as it was last year.)
I found more blackberries than you. (= than you found.)
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subjects: more than is necessary; as happened
He worries more than is necessary. (NOT ... more than it/what is necessary.)
The train might be late, as happened yesterday. (NOT ... as it happened yesterday.)
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Why should sometimes subjects be omitted, sometimes not?
The weather isn't as good as it was last year.
He worries more than is necessary.
kadioguy Why should sometimes subjects be omitted, sometimes not? You posted the answer yourself: We can leave out words if the meaning is clear.
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kadioguyWhy should sometimes subjects be omitted, sometimes not?
You posted the answer yourself: We can leave out words if the meaning is clear.