"你应该答应她”用英文怎么说？

应该翻译成：
You should answer her.

但是从你的意思看，我估计你想说你应该答应她，估计就是答应她的要求什么的，所以可以翻译为：
You should agree with her.或者You should say ok to her.

看你结合前后文怎么选择了。

You should have promised her

you should promise her.
or: you should say yes to her.

you should say "yes" to her.

You should promise to her.

you should promise her.