In grammar, a modifier is an optional element in phrase structure or clause structure. A modifier is so called because it is said to modify (change the meaning of) another element in the structure, on which it is dependent.Typically the modifier can be removed without affecting the grammar of the sentence.ometimes it is not clear which element of the sentence a modifier is intended to modify. In many cases this is not important, but in some cases it can lead to genuine ambiguity. For example: 1  He painted her sitting on the step.