If we substitute something (let’s call it A), that means A is either: Being used or placed in the place of something else (B). In this case, we use the preposition for. The sentence pattern is: substitute A for B, and means that in the end A is used instead of B. Having something else (B) being used or placed in its place.

Synonyms for THIS: closer, near, front, inside, hither, nigher, fore, forward; Antonyms of THIS: that, other, further, opposite, far, farther, distant, remote