A classmate of mine always corrects my grammar, whether we’re just chatting before class or talking amongst a group of friends at lunch. To make matters more annoying, she’ll throw in a Latin lesson so that I can understand how bad my blunders are. How do I let her know that it offends me?
As graciously as possible, thank her for her gesture. Then say, “I understand that you’re trying to help me, but your remarks and your corrections wear me down. I’d love to just have a conversation without grammar and Latin lessons.” However, if she’s truly a complete pain, you may want to just let it slide and be the better person; maybe she could correct your paper that’s due in a few weeks.
Besides, a small lesson every now and then can keep us students on our toes; we’re never done learning, no? Whether she’s one of those many English majors or a crazy classics student, she’s probably helping you more than you realize.