Surely / certainly

I’m thinking of how to tell them apart…

the cambridge advanced learner’s dictionary :



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 ?

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.

Many Thanks!