然而， 在另一些場合里， unless并不是表示 if...not的涵義， 而是表示近似于but...if的涵義.這時候， 可以改寫為but...if， 但決不能用if...not去替代. 例如， 衽在法庭上所說的那一句， 可以改寫為:
I could not have got to Slough， but I could have got to Slough if I had had a helicopter.
還有一點應當注意的是， unless句型和if...not句型具有沒的倏色彩和表達風格，前者是積極的說法， 態度明朗， 語氣有力; 后者是退一步的說法， 態度含糊， 語氣軟弱無力.
現在， 你能否說出下列兩個句子的不同涵義?
I should not expect their help unless they were my friends.
I Should not expect their help if they were not my friends.
