Adverbs of certainty express how certain we feel about an action or event. Adverbs of certainty go before the main verb unless the main verb is 'to be', in which case the adverb of certainty goes after.
- He definitely left the house this morning.
- He surely won't forget.
- He is probably in the park.
- He is certainly a smart man.
If there is an auxiliary verb, the adverb of certainty goes between the auxiliary and the main verb.
- He has certainly forgotten the meeting.
- He will probably remember tomorrow.
- He is definitely running late.
Sometimes these adverbs of certainty can be placed at the beginning of the sentence.
- Undoubtedly, Winston Churchill was a great politician.
- Certainly, I will be there.
- Probably, he has forgotten the meeting.
When the adverb of certainty surely is placed at the beginning of the sentence, it means the speaker thinks something is true, but is looking for confirmation.
- Surely you've got a bicycle.
- Surely you're not going to wear that to the party.