i will tell you where to get off是单句还是从句,为什么?还有where to get off是一个固定词组吗?

这是个宾语从句,只是它省略了宾语从句的标志that.这个句子也可以写成I will tell you that where to get off.因为我们知道在宾语从句中,that是可以省略的.where to get off是I will tell you的宾语.
如果你要弄清楚宾语从句的主谓宾,不访我们把这个句子还原,I will tell you that I get off to where.
不知道你明白了没?