Subjective complaints are still important data for restaurants. If one person doesn’t like the new hot-dog lasagna, it’s an aberration. If 15 people don’t like it, the recipe probably needs to go back to the workshop. The chef or owner needs to know this, but also needs to hear it in terms that won’t lead to a screaming match in the dining room. Perhaps you have seen Gordon Ramsay critique another chef’s cooking on television? Don’t do it like that.

In general, the more specific your complaint, the more likely it is to be understood. The worst, most useless and potentially dangerous complaints are broad, sweeping condemnations.

“There is complaining that makes you think about what you’re doing, and there is complaining where everybody thinks they’re entitled to say anything,” said Rita Sodi, the chef and owner of the Tuscan restaurant I Sodi in Manhattan. “Saying, ‘This is terrible’ is not complaining. That is being rude. It’s like, ‘You’re ugly.’ It’s telling me that I’m ugly. It’s personal. It’s my food.”

Even when the person you’re grousing to did not cook your pasta personally, you should proceed gently, in nonconfrontational terms. It may be helpful to imagine that you are speaking with an air traffic controller trying to land 20 jets during a snowstorm; you would try very hard not to add to the overall stress level in the tower, even if your child was on one of those jets.

“If you can be patient, open and polite, that’s really helpful,” Ms. Shopsin said. “The more collaborative and open you can be — ‘Excuse me, I’m sorry’ — even though it’s not your fault that it got messed up. People have to understand that sometimes to fix a mistake is not easy.”

And sometimes, a restaurant can make customers happy by changing something that isn’t a mistake at all. Occasionally, diners at Shopsin’s who had ordered migas complained about finding cilantro in the dish. Initially, the response would be that migas are supposed to have cilantro. Which may be true, but perhaps wasn’t the point.

Now, Ms. Shopsin asks customers whether they like cilantro. If the answer is no, there won’t be any in their food, even if, strictly speaking, it belongs there.

“Complaints do help change things,” she said.