选择正确的引导词补全句子

1.I don't know ________ he will come.(that,if)

2.I believe ________ he is a good teacher.(that,if)

3.I want to know ________ you join this club.(why,what)

4.They asked ________ I came from.(if,where)

5.I can't remember ________ I bought this bag.(when,that)