Surely / certainly

English as a Second Language English Vocabulary, Grammar and Idioms
1

I’m thinking of how to tell them apart…

the cambridge advanced learner’s dictionary :

surely
http://dictionary.cambridge.org/define.asp?key=80176&dict=CALD

certainly
http://dictionary.cambridge.org/define.asp?key=12279&dict=CALD

in “basic english usage” michael swan writes :

Which explanation is right?
I know that surely expresses surprise, certainly does not.
In elem #15 “an easy question” q5:

Does certainly fit in here ?

2

I have used surely in this sentence because here it has the meaning: You must agree with me - you really must. In a way it’s asking for reassurance from the other person of what the speaker is saying.

3

Many Thanks!