英语语法高手请进

1. 改正：It is necessary that he (should) get every thing ready by Wednesday.

2. 此种改法的确是使用虚拟语气：固定句型“It's necessary/important/a pity/...+that从句”中，that所引导的主语从句可以使用虚拟语气，即从句谓语动词由“should + 动词原形”构成，其中should可以省略。
当然，这种结构不用虚拟语气也可以，判断依据是看说话人的语气强烈不强烈。当然这样说有点算是废话，不过楼主记住这样的句型可以使用虚拟语气就可以了。
反过来说，原句使用一般现在时he gets...的确不妥，句意也不太通顺。

3. by是介词，意思是“不迟于”= not later than.
by Wednesday意为“不迟于周三”，即“最晚周三前”。

1、不知道改是什么意思
改成简单句的话是：It is necessary for him to get everything ready by wednesday。
2、不用虚拟语气
判断虚拟语气的话，就要牢记虚拟语气的几种形式，还有特殊的虚拟就要因句子而异了、
对现在虚拟 ：If sb. did sth., sb. would do sth.
典型句子： If I were you, I would do the same thing. 注意：不管什么人称，Be动词都用were
对过去虚拟： If sb. had done sth., sb. would have done sth.
对将来虚拟：If sb. were to\ should（第一人称）\would do sth.，sb. would do sth.
3、wednesday前面加By指的是，在wednesday前面
by 加一个时间点是指在这个时间之前。

by+时间点(已发生某事) 多用过去完成时，
by Wednesday在星期三的时候
不应该用虚拟，虚拟语气是指不可能发生的事，
但句中不知道他有没有做好准备所以
用it was necessary that he (should) get everything ready by Wednesday.

It is necessary that he gets everything ready by Wednesday.

He needs to have everything ready by Wednesday.

by Wednesday 就是星期三之前的意思。是陈述句，没有虚拟。

用手should型虚拟语气。就是It is +necessary /possible/probable ...that (should) do ...

by就是在。。。之前