Generally used to express how strongly we believe or feel about something.
Probably, clearly and certainly are examples of adverbs of certainty.
With a few exceptions these types of adverbs are positioned towards the middle (after the auxiliary verbs and before other types of verbs).
He clearly won the competition.
He is certainly a talented player.
Two exceptions to these rules are may be and perhaps. These generally go at the beginning of a clause.
Use caution with the adverb of certainty – surely. If this is at the start of a sentence, it usually indicates that the person believes something, but wants a confirmation that it is so.