1）句子结构的省略
1.在when, while, whenever, till,as soon as, if, unless, as if, though, as, whether引导的状语从句中，若谓语有be动词，而主语又跟主句主语相同或是it时，则主句的主语和be常可省略。
Though (he was)tired, he was not disheartened.
2. 在限制性定语从句中，作宾语的关系代词whom,who,which,that可省略。The girl (who\whom\that)the teacher spoke to is Liu Ying.。
3. 在know,think,consider,suppose,find,believe,say,decide等动词后的宾语从句中that可省略；若带有多个宾语从句，只有第一个可省略，其余的不可以省略。
4. 在与suggest\request\ order\advise等词相关的名词性从句中必须用虚拟语气。“should + v”形式，should可省略。The officer ordered that his men (should) fire.
5.省略一个从句或从句的一部分，可用so 或 not代替。常用于此类的动词有believe , think, expect, guess, hope, imagine, suppose, say, tell等以及appear, seem, afraid等。--- Is he feeling better today ?
--- I’m afraid not. (not = he feeling better today)
6.虚拟条件句谓语有were\had\should时，可省略if，把were\had\should移到主语之前,Were I you, I would do the work better.
7.动词不定式的省略
2）只保留to的场合
a.不定式作某些动词的宾语，宾补或主补时，常见动词：like, love, care, hope, wish, expect, prefer, refuse, mean, try, advise, persuade, agree, want, afford, forget, remember, manage等。
She wants to come but her parents won’t allow her to.、
b.作某些形容词的状语时，常见的形容词：happy, glad, eager, anxious, willing, ready等。I think she should get a job, but you can’t force her if she’s not ready to.
c.常见结构如：be able to , be going to, have to, ought to, used to 中。但当不定式是作助动词用的have或be的形式时，要保留到原形have或be. He did not come, but he ought to have (come).
He is not the man he used to be.
3)不定式符号to的省略
1. 主语部分有to do, 系动词是is\was时，作表语的不定式省略to。
The only thing you have to do is (to ) press the button.
2. 作介词but, except, besides的宾语，前面有实义动词do的各种形式时，常省to.
Tom had nothing to do besides answer letters this morning.今天上午汤姆除了复信什么也没做。
3. 当两个或多个不定式并列时，其后的不定式符号可省略。但有对比关系时不可省。It is easier to make a plan than to carry it out.
4. 在see, watch, notice, hear, listen to, look at, feel, have, make, let, leave, observe等词后作宾语补足语时和would rather…than…why (not) do，had better结构后，省略to.
参考资料：http://wenku.baidu.com/view/65783c23dd36a32d737581cf.html