ENGLISH unless 可以引导真实条件句和非真实条件句有时可以和IF...NOT 替换,有时可以和BUT..IF

ENGLISH
unless 可以引导真实条件句和非真实条件句有时可以和IF...NOT 替换,有时可以和BUT..IF...或except on condition that .替换.
要怎么区分呢?

一、if 和 unless 根据说话者的言外之意,可以引导两种语气的条件句：
1.真实条件句
She will play the piano only if she is paid.只有付给她报酬,她才愿意演奏钢琴.（付给报酬是可能的）
Unless he studies hard,he will never pass the examination.
他如果不努力学习,就永远不能考及格.（其实他就是不努力学习）
2.虚拟条件句
If I were to go,I would be late.若我去,就要迟到.（根本就把打算去）
I should go unless it rained.如果不下雨我就去.（雨没有停止的可能）
二、unless = if not,因此无论是真实的还是虚拟的,后者可以用来取代前者,如上面句子的改写：
If he doesn't study hard,he will never pass the examination.
I should go if it didn't rain.
三、but that 和 but for 意思是“若不是”,只用于虚拟语气,如上面虚拟条件句的进一步改写：
I should go but that it will rained.（注意：因为确实要下雨,but 从句本身不虚拟,但主句需要虚拟.）
I should go but for raining.
另如下面例句的反向改写(注意以下各句从句的时态)：
They would have resisted but that they lacked courage.如果不是缺乏勇气她们会抵抗的.
They would have resisted unless they had lacked courage.
They would have resisted if they had not lacked courage.
四、except that / except for / except on condition that 是对主句细节的修正,只用于真实语气,如：
She will play the piano only if she is paid.可以改写为：
She will play the piano except that she will be paid.
She will play the piano except on condition that she will be paid.