條件句為在「未來」有可能發生的情況條件，但不確定是否成真。標記上，以沒有特定時間標記的「習慣式」表達「可能的條件」，以「未來式」表達「未來結果」：
If you work hard, you will get a good grade.
If it rains tomorrow, we will cancel the outing.
假設語氣亦即「與事實相反」的假設，所謂「事實」，是「真實已知的情況」，必然在現在或過去出現。「與事實相反」的假設就是與「目前狀態」或「過去事實」相反之希望。既與事實相反，在時態上就得和表達「事實」的標記有所區別，英文選擇用「錯亂」（將時態倒退）來標示「違反事實」。
違反現在事實 → 以過去式標記
現在事實：She is a girl.
違反現在事實：I wish she was/ were a boy. （were 式形式固定的「反事實」標記，不隨人稱變更）
造句：If she were a man, she would join the army.
違反過去事實 → 以過去完成式標記
過去事實：She didn't come home yesterday.
違反過去事實：I wish she had come home yesterday.
造句：If she had come home yesterday, she would have enjoyed the dinner with us.
Related question: 條件句和假設句與事件真實性有何關聯？
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