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.
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English as a Second Language
You can read my ESL story Present Simple

[Michael]

Many Thanks!