Surely / certainly

[Michael]

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 :

[Alan]

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.
_________________
English as a Second Language
You can read my ESL story The way you write it

[Michael]

Many Thanks!