A good grammar checker should pick up various mistakes including subject-verb agreement and using the wrong homonym. The finer points of grammar can get lost in a creative frenzy, plus many talented writers were bored in English class and didn’t learn as much as they should have about proper use of the language. One mistake I hear a lot is “we was”.

Another mistake waiting to happen is when a sentence has something like a preposition phrase between the subject and verb, it’s easy to mess up. For example, “He looked over his list of grocery items, which was written on the back of his hand” (correct where “list” is what “was written”) versus “He looked over his list of grocery items, which were written on the back of his hand” (incorrect, i.e. “items” isn’t what’s written on his hand).

One homonym frequently used incorrectly, which drives me nuts every time, is using shutter (a window covering or decoration) instead of shudder (to shake or shiver with fear or cold.)  Look it up!