1.當if…not…引導非真實條件分句時，一般不可改用unless。 例如： If she had not had an alarm clock,she would have missedthe train.（事實：她有鬧鐘。） 如果把上述例句中的if…not…換成unless，意義就相反了。 （誤）Unless she had had an alarm clock, she would havemissed the train.（事實：她沒有鬧鐘）
2.If…not…引導的條件分句可以重疊使用，unless則不可， 因為unless表示唯一條件。例如： She won't lose weight if she doesn't keep a diet and ifshe does not take exercise every day.
3.unless引導的分句可用否定結構，而if…not …引導的分句不可再否定。例如： Don't ask me to explain unless you really don't understand. The directors have a meeting every Friday,unless there is nothing to discuss. 以上兩例句中的unless不可換成if…not…。
4.在unless引導的肯定形式的條件分句中， 通常用肯定詞（即some,already,still,too,as well等），在if…not…引導的分句中，通常用非肯定詞（即any,yet,ever,either,at all等）。例如： I will criticize you if you haven't spoken to her yet. I will criticize you unless you have already spoken to her. （誤）…unless you have spoken to her yet. 不過在否定形式的非真實條件分句中，if…not …結構仍能與肯定詞連用。例如： If he hadn't already prepared his lesson, he wouldn't beallowed to go.（因為這種從句表面上是否定，實際上是肯定。）
5.在間接引語中，當if=whether，意為“是否”，unless 不可以代替if…not…。例如： She promised to let me know if she wasn't coming.
6.當unless含有“除了”的意思，不可為if…not…所替代。例如： He never stammers,unless when he is angry.( unless when=except when) Nothing will come out of it, unless disaster.(unless=except)
7.當主句是疑問句時，不可用unless代替if…not…。 因為unless用在A情況發生，必導致B結果之類的複合句中。例如： What shall we do it they don't reply to our letters?
8.當主句謂語是be＋某些形容詞或分詞，如：glad,surprised,disappointed,satisfied,ashamed,delighted和pleased等， 不可用unless代替if…not…。例如： I'll be really surprised if they don't come to the meeting. I'll be quite glad if she doesn't join them. 之所以不能為unless代替，是強調否定條件的出現，表明在這種前題下的因果關係。以上條件狀語從句都可變為原因狀語從句。例如： I'll be really surprised that they won't come to the meeting. 這裡if被that代替了，而從句的否定結構不變。 綜上所述，unless既等於if…not…，在某些場合又不相等， 因此要牢記它們之間的差別，準確地使用它們。
unless在意義上相當於if…not…，因此在有些場合if…not…可為unless所代替。但在有些場合又不能互換，我們要注意下面幾種情況：
1.當if…not…引導非真實條件分句時，一般不可改用unless。 例如： If she had not had an alarm clock,she would have missedthe train.（事實：她有鬧鐘。） 如果把上述例句中的if…not…換成unless，意義就相反了。 （誤）Unless she had had an alarm clock, she would havemissed the train.（事實：她沒有鬧鐘）
2.If…not…引導的條件分句可以重疊使用，unless則不可， 因為unless表示唯一條件。例如： She won't lose weight if she doesn't keep a diet and ifshe does not take exercise every day.
3.unless引導的分句可用否定結構，而if…not …引導的分句不可再否定。例如： Don't ask me to explain unless you really don't understand. The directors have a meeting every Friday,unless there is nothing to discuss. 以上兩例句中的unless不可換成if…not…。
4.在unless引導的肯定形式的條件分句中， 通常用肯定詞（即some,already,still,too,as well等），在if…not…引導的分句中，通常用非肯定詞（即any,yet,ever,either,at all等）。例如： I will criticize you if you haven't spoken to her yet. I will criticize you unless you have already spoken to her. （誤）…unless you have spoken to her yet. 不過在否定形式的非真實條件分句中，if…not …結構仍能與肯定詞連用。例如： If he hadn't already prepared his lesson, he wouldn't beallowed to go.（因為這種從句表面上是否定，實際上是肯定。）
5.在間接引語中，當if=whether，意為“是否”，unless 不可以代替if…not…。例如： She promised to let me know if she wasn't coming.
6.當unless含有“除了”的意思，不可為if…not…所替代。例如： He never stammers,unless when he is angry.( unless when=except when) Nothing will come out of it, unless disaster.(unless=except)
7.當主句是疑問句時，不可用unless代替if…not…。 因為unless用在A情況發生，必導致B結果之類的複合句中。例如： What shall we do it they don't reply to our letters?
8.當主句謂語是be＋某些形容詞或分詞，如：glad,surprised,disappointed,satisfied,ashamed,delighted和pleased等， 不可用unless代替if…not…。例如： I'll be really surprised if they don't come to the meeting. I'll be quite glad if she doesn't join them. 之所以不能為unless代替，是強調否定條件的出現，表明在這種前題下的因果關係。以上條件狀語從句都可變為原因狀語從句。例如： I'll be really surprised that they won't come to the meeting. 這裡if被that代替了，而從句的否定結構不變。 綜上所述，unless既等於if…not…，在某些場合又不相等， 因此要牢記它們之間的差別，準確地使用它們。