When Jeff Cohen was getting ready to meet his OkCupid date for drinks in Manhattan, he started to have second thoughts as he reread the glaring grammatical error in her last message: “I will see you their.”

The date flopped for a couple of reasons, but bad grammar bothers Mr. Cohen. Learning a potential mate doesn’t know the difference between “there,” “they’re” and “their” is like discovering she loves cats, he says. Mr. Cohen is allergic to cats. “It’s like learning I’m going to sneeze every time I see her,” he says.

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