that:I think that you can pass the exam.Whether/if:I don’t know what the word means.“Wh”:I don’t know what the word means.I don’t know where he found the book.
只用whether的情况：1.与or not连用：I don’t know whether it’s raining or not.2.与动词不定式连用：He doesn’t know whether to accept the invitation.3.连接词前有介词时：It depends on whether he is coming.
1.主句是一般现在时态，从句根据实际情况而定(各种时态均可)She wants to know what he has done for the exam.
2.主句是一般过去时态，从句用相应的过去的时态。1)She said that she was a student.2)She said that she would fly to Japan in a week.3)She said that she had finished her homework already.
3.如果宾语从句说的是客观真理、自然现象或事实时，这时宾语从句要用一般现在时态。The teacher said that the earth goes round the sun.
1.This is the teacher whomwho we like best.2.I don’t like the boy to whom you are talking.
Whose:指人或物，作定语，表示“…的”eg:Harry is the boy whose mother is our math teacher.
1.先行词前有序数词修饰时：This is the first gift that my parents bought me.
2.先行词前有形容词最高级修饰时：This is the most exciting film that I have ever seen.
3.先行词是不定代词something,anything等时.e.g.Is there anything that you want in this shop
4.先行词是人和物时,用that.e.g.He talked about some writers and books that were unknown to us all.
5.先行词被all,little,the only,the very(就是，正是),the last等词修饰时,只能用thate.g.This is the last place that I want to visit.
6.特殊疑问句以who或which开头，只能用that引导.Who is the girl that is making a speech on the platform?
当关系代词前使用介词时：e.g.1.This is the train by which we went to Beijing.2.This is the teacher to whom my mother is talking.