Student or Learner
In the sentence : "I feel certain that she will be successful", does the adjective "certain" modify the word "I" or "feel" ? Actually, I think it modifies the word "I", but I'm not sure.
And does the adjective "successful" modify the word "she" ?
Technically, the adjective doesn't modify anything here: the term 'modification', properly applied, refers (with regard to adjectives) to the relation between attributive adjectives and their nouns.
This adjective is part of the predicate, and as such relates to the subject pronoun 'I'. It also complements the copular verb 'feel'.