the adverb form of ‘certainly’ is used to indicate strong assurance. It is commonly used to introduce or emphasize an assertion.
1. 'I certainly understand your feelings on the matter.'
2. 'She certainly won’t be happy about this.'
3. 'He certainly handled that situation better than I thought he would.'
while ‘certainly’ carries much of the same assurance as ‘sure’ or ‘definitely’, it should not be overused in order to support the strength of the assertion. Also, note that it does not always have to follow directly after the part of speech it is modifying.