* 主詞 + should have + 過去分詞, but + 主詞+ 過去式動詞
I should have called you yesterday, but I forgot.
* 主詞 + would have + 過去分詞, but + 主詞+ 過去式動詞
I would have finished the work last night, but I was too tired.
* 主詞 + could have + 過去分詞, but + 主詞+ 過去式動詞
He could have helped me, but he was too lazy.
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