刚看到alex119网友已做了很好的回答,我认为简明扼要,应予以采纳。
我是才看到邀答私信,正好之前收集过这一问题的资料,然后就改写了一下。既然写了,就发一下吧。
句① lf he be found guilty, he would be sent to prison.
本句中从句的be是虚拟语气现在时,从句的would表明作者认为可能性很小。整句为非真实条件句。
但分句和主句动词形式的这种搭配很少见,是一种古旧用法,传统英语语法(尤其是国内英语语法学者)对此时有提及。例如:
If his advice be good, it would be accepted. (张道真《英语语法大全》)
但现代英语语法对此已少有着墨。
当然,从句动词采用be-型虚拟式并不意味着全句必然是非真实条件句中。因为跟should do/be一样,be-型虚拟式(实际上可简单视为should be/do)既可以是真实条件句的标记,也可以是非真实条件句的标记。如果主句用shall, will,一般现在时,或者是祈使句等,全句就是真实条件句。比较而言,后一种用法(真实条件句)更常见。例如:
If a sever serve a fault twice, he shall lose a point. (新编高级英语语法)
If it be asked what the subjunctive in the above instances expresses, the answer is threefold.(HEG)
If it be inappropriate to have said this, I humbly apologize. (Royal English Grammar)
If she be here, then tell her I wait her pleasure. (PEU)
句② lf he should come tomorrow, l would tell him everything.
由主句的would可知,本句为非真实条件句。从句的should为推定式用法,具有试探性、不确定性,比用一般过去时came更委婉圆通,也更正式。这类用法很常见。例如:
If John should come, Mary would leave. (MEM)
也可将should前置,构成部分倒装语序。例如:
Should John come, Mary would leave.
句③ I'm writing to confirm the arrangements for my trip to China. I should be leaving New York Airport at 8 p.m. on July 18.
句中should be leaving用于对将来事件的推测(预见),表示较高程度的可能性,即非承诺必然性(noncommitted necessity)。其确定性介于may和will之间。Leech认为其概率相当于probability。例如:
You should be meeting those later on this afternoon. (MEM)
当然,正如静心网友指出的那样,will/shall be doing也可用于对将来事件的推测。例如:
She will be arriving tomorrow. (CGE)
一般来说should be leaving和will be doing基本语义相当,但后者可能性更大一些。
以上供参考。