Little John keeps borrowing Bob's colored pencils but he's too playful to remember to give them back. Bob's mom is upset and talks to John's about it. John's mom would say something along the lines of either
He just borrows them, not steal.
He just borrows them, not steals.
Which one is grammatically correct? Why?
I know it's feasible (and probably safer) not to omit anything and go with two full sentences. On the other hand, sometimes we need to emphasize the contrast between the two verbs, or need to be simply more concise. Consider another example, in the legal sense of the verbs, without omission of the second object:
Your argument excuses his actions, not justify / justifies them.
If I'm honest, the latter option seems clunky, but then how would we denote subject-verb agreement, and why wouldn't a simple omission of subject (without affecting the grammar of the rest) work?