T：First I will test how well you understand what you learned yesterday.Please judge whether the following sentences are right.If they are not right，please correct them.

　　（1）It is with the help of the teacher that I passed the exam.

　　（2）It is I who is wrong.

　　（3）It was she that he helped with her homework yesterday.

　　（4）It was at the post-office where we met each other.

　　（5）It was yesterday afternoon when they played a close basketball game.

　　（6）Who was it that discovered the secret？

　　S：I think it's right.

　　T：Do you agree with him？

　　S2：No，I think it is not right. But I don't know why.

　　T：Does any one know the reason？

　　S3：In that cause，the past tense is used，so I think we should change "is" to "was".

　　T：Quite Good. That is to say：当原句的时态为表示现在的各种时态时，用It is...；当原句的时态为表示过去的各种时态时，则用It was...。Now，what about the second sentence？

　　S：I think it is right.

S：I don't agree with her. Here "who" refers to "I"，so after I we should use "am".