You use surely to emphasize that you think something should be true, and you would be surprised if it was not true.
You're an intelligent woman, surely you realize by now that I'm helping you.
你是个聪明的女人,现在肯定明白了我是在帮你。
You surely haven't forgotten Dr Walters?
你肯定还没忘记沃尔特斯博士吧?
If something will surely happen or is surely the case, it will definitely happen or is definitely the case.
He knew that under the surgeon's knife he would surely die.
他知道自己必定会死在外科医生的刀下。
slowly but surely
If you say that something is happening slowly but surely, you mean that it is happening gradually but it is definitely happening.
Slowly but surely she started to fall in love with him.
虽然过程很缓慢,但她确实开始爱上他了。