These adverbs express how certain or sure we feel about an action or event.
Common adverbs of certainty:
certainly, definitely, probably, undoubtedly, surely
Adverbs of certainty go before the main verb but after the verb 'to be':
He definitely left the house this morning.
He is probably in the park.
With other auxiliary verb, these adverbs go between the auxiliary and the main verb:
He has certainly forgotten the meeting.
Sometimes these adverbs can be placed at the beginning of the sentence:
Undoubtedly, Winston Churchill was a great politician.
BE CAREFUL! with surely. When it 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?