Adverbs of certainty express how certain or sure we feel about an action or event.

Common examples are: certainly, definitely, probably, undoubtedly, clearly, obviously etc.

• He is undoubtedly a great leader.
  
• There is clearly something wrong.
  
• She is definitely taller than you.
  

Position

Adverbs of certainty usually go in mid position. They are placed after auxiliary verbs and before other verbs. When there are two or more auxiliaries, the adverb goes after the first.

• He is undoubtedly a great leader. (is/am/are/was/were + adverb)
  
• She will probably come. (auxiliary verb + adverb + main verb)
  
• It will certainly rain this evening. (auxiliary verb + adverb + main verb)
  
• I certainly feel better today. (adverb + main verb)
  
• You have definitely been working too hard. (first auxiliary + adverb + second auxiliary + other verb)
  

Exceptions

Perhaps and may be are exceptions to this rule. They usually go at the beginning of a clause.

• Perhaps she will come.
  
• May be you are right.