What's the matter if I add [and it was] there? Would this sentence become incorrect?
[The idea lit up her heart (and it was) like a flash of lighting]

Question

Answer

It would change the meaning. Without 'and it was', her heart lit up like a flash of lightning. When you include 'and it was', 'it' refers to the idea - so the idea was like a flash of lightning, which doesn't make much sense.

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