I saw the following sentence:
Sally, who I saw in town yesterday, is a friend of my sister.
But I think it should use whom instead of who, since Sally is the guy I saw, so why not use whom?
I saw the following sentence:
Sally, who I saw in town yesterday, is a friend of my sister.
But I think it should use whom instead of who, since Sally is the guy I saw, so why not use whom?
You're correct; it should be "whom." It's just that the word "whom" has fallen out of favor and isn't always used. Modern style guides accept "who" in all but a few specific cases.
Sally, who I saw in town yesterday, is a friend of my sister.
PS: Modern stylists now suggest that it is also ok to use who when uncertain. But in your example, it is definitely 'who'.
You can check out some reputed sites such as grammarly which elucidate the usage of who and whom:
The Difference Between Who and Whom
How can you tell when your pronoun is the object of a verb or preposition? Try substituting “he” or “she” and “him” or “her.” If “he” or “she” fits, you should use who. If “him” or “her” fits, you should use whom. Keep in mind that you may have to temporarily rearrange the sentence a bit while you test it.
Who/whom ate my sandwich?
Try substituting “she” and “her”: She ate my sandwich. Her ate my sandwich. “She” works and “her” doesn’t. That means the word you want is who.
Whom ate my sandwich?
Who ate my sandwich?