请教英语语法-虚拟语气的使用方法.

虚拟语气中有一种叫着虚拟条件句（或叫着含蓄条件句）,而非真实条件句中的条件从句有时不表达出来,只暗含在副词、介词短语、上下文或其他方式表示出来,这种句子叫做含蓄条件句,在多数情况下,条件会暗含在短语中,如without…,but for…,in the absence of···,otherwise等.这些句子中的结构一般如下：
主句：主语+should/would/could/might+do/have done.
从句,but that/otherwise + 主语 + 过去时态
eg：
But for his help,we would be working now.　　要不是他的帮助,我们还会在工作呢.
Without your instruction,I would not have made such great progress.　　要是没有你的指导,我不会取得如此大的进步.
We didn't know his telephone number,otherwise we would have telephoned him.　　我们不知道他的电话号码,否则我们就会给他打电话.
in the absence of sunlight,there would be no moonlight.如果没有太阳光,就不会月光.
but表示让步关系,意思是“除非,要不是”,常与that一起构成but that,相当于"if...not".例如：I would have failed but that your helped me.要不是你们帮助我,我就会失败.
其他不懂的给我留言吧,这个我擅长.