www.alphadictionary.com

Expressions of certainty, sureness and doubt are not always what they seem on the face of it. There are nuances of meaning, which must be confusing for non-native speakers. For example:

1. He will certainly come.
or
I am telling you for certain that he will come.
(= he will definitely come; I know it)

2. I am certain that he will come.
(= I don't know it, but I have no doubt about it)

3. He will no doubt come.
(= I am confident that he will come, but there is a very slight element of doubt)

4. He will surely come.
(= there is a greater element of doubt)

5. Surely he will come.
(= I am fairly confident that he will come, but I am slightly worried that he may not)

6. Surely he's not going to come [?/!]
(= Oh God, what if he does?)

Is it possible that some of these distinctions are idiosyncratic or regional? Once, on My Word! - which, being British, you may be familiar with - there was a question about the difference between ‘vengeance’ and ‘revenge’. The answer being that vengeance is revenge in kind, while revenge is just getting even. The OED definitions offer no such understanding.

Another point is that intonation makes a big difference in the certainty conveyed. Which word is stressed also changes things. The subject line is all of 4 words, but it's meaning changes if you put the questioning stress on any of them.