They are a cancer to society.
Could you tell me If it's grammatical or not? If grammatical, Why not "They are cancers to society" but "They are a cancer to society"?
The subject complement does not have to agree with the verb or subject. That might be what is troubling you about this example. I don't know the reference for "they" in this case, but it might refer to the members of a group who are all doing the same thing that is undesirable.
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The subject complement does not have to agree with the verb or subject. That might be what is troubling you about this example. I don't know the reference for "they" in this case, but it might refer to the members of a group who are all doing the same thing that is undesirable. As a group, they are a cancer. If they were cancers, there would be more than one problem or attack to the system.