if/whether引导的从句

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if/whether引导的从句
谁能给我说一下if/whether引导的宾语从句（解释为是否）
它们的时态怎么用？我们老师说用“退步法”谁能给我详细易懂的解释一下？最好有例句。
谢谢！

1.当主句是一般现在时，从句可根据需要用任意时态.
The teacher wants to know if/whether the students are studying in the classroom now.
The teacher wants to know if/whether the students have finished their homework.
The teacher wants to know if/whether the students were playing football yesterday afternoon.
2·主句用过去时，从句用过去时态的一种。
He asked whether /if Tom would come to his party.
The mother wondered whether /if his son had finished his homework.
3·主句用过去时，从句是真理时，只用一般现在时
He asked whether /if the earth goes round the sun.
if,whether在宾语从句中的区别
1.介词后的宾语从句只能用whether引导,不能用if引导.
They are talking about whether they will be paid very soon.
2.whether后可以加or not,但是if不可以.
3. 在不定式前只能用whether.
He doesn't know whether to help her.

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