In grammar or English language, Substitution is a term that describes the process of changing or replacing some specific words or phrases in a sentence to give the same meaning as intended while also preventing or avoiding repetition of the original words or phrase.
Modulation on the other hand is a conversation strategy in which some words or phrases we're pronounced or stressed in a particular way, to give the intended meaning from the usual meaning.
Repetition helps exaggerate a point that the author is trying to convey. It highlights the main idea of what that character in the story is trying to let readers understand when stating such phrases.
