"我知道他是老师"是 I know he is a teacher。后半句本身是个完整句子，当了 know 的宾语，这就是宾语从句。
三种引导词
that：陈述句变宾语从句，that 常省略
- I think (that) he is right.
- She said (that) she would come.
if / whether：一般疑问句变来，"是否"
- I don't know if he will come.
特殊疑问词：特殊疑问句变来
- I don't know where he lives.
- Can you tell me what this is?
语序铁律：从句永远陈述语序
不管原句是不是疑问，塞进从句后都改回陈述语序：
- 错：I don't know where is he.
- 对：I don't know where he is.
- 对：I wonder if she is happy.
时态呼应
主句过去时，从句也退一档：
- He says he is busy. / He said he was busy.
客观真理永远现在时：The teacher said the earth goes around the sun.
口诀：that 陈述 if 疑，疑问词照抄别跑题；从句语序陈述排，主过从退真不变。
