1：But for his help,we would be working now.
要不是他的帮助,我们还会在工作呢.还在工作就应该表示正在工作,所以用be+动名词
2：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.
3：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.
第二第三句都是事实上这个是和过去的真实情况相反的,所以我们用虚拟语气,同时它又是表示过去的时间的所以我们用would have done的形式