1.that引导主语从句常置于句首，从属连词that在从句中不作任何成分，也没有实际意义，只起连接从句的作用。 That he will come to the conference has excited every one of us. That in some countries women are still treated unequally is unfair.
2.it可代替主语从句置于句首作形式主语，常见的it替代主语从句的句式有： （1）It+系动词+形容词（necessary，right，likely， unlikely，wrong，important，certain，clear，obvious，strange，natural等）+that从句 It is certain that most of the farmers have bought in more money by all means. 注意：在“It is necessary，important，strange，natural+that从句”结构中，从句常用“（should+动词原形”形式。 It is necessary and important that one(should)master the skill of operating computer so as to meet the need of a job. （2）It+be+名词（短语）（a pity，a shame，good news，a fact，an hour，a wonder，no wonder等+that从句 It’s no wonder that you’ve achieved so much success. （3）It+be+过去分词（said，told，heard，reported，decided，suggested，advised，ordered，remembered，thought，considered，well known，announced等）+that从句 It is said that the professor has already succeeded in carrying out this experiment. 注意：在It+be +suggested，advised ，ordered， requested ，insisted ，required+that从句中应用“(should+动词原形)”。 It was decided that they (should+) start the project the next month. （4）It+特殊动词（seem，appear，happen，matter）+that从句 It happened to me that I had been away when he called.
3.that主语从句的疑问形式和句子的一半疑问形式一样。 Was it decided that they(should)start the project the next month? Did it happen to you that you had been out when he called?