首先记住：have to do sth 是实意动词或行为动词.
I have to do the housework every day.I had to...
I don't have to do the housework every day.I didn't have to...
Do you have to do the housework every day.Did you have to...
Yes,I do.Yes,I did.
No.I don't.No,I didn't.
另一种情形是作为 ---Must...的否定应答之一：
---Must I hand in my homework right now?
--- Yes,you must.
---No,you needn't = you don't have to.You may hand it in this afternoon.