Q. My 20-month-old son is well behaved, but I don't know what to do about his manners.

Now that he is starting to speak, should we be teaching him to say "please" and "thank you"?

I don't want to insist on good manners before he's ready, but I do want him to be respectful and polite. How can I teach him to do that?

A. Some people think good manners don't matter, but they're wrong. Manners are not only the universal sign of kindness and respect, but they also are the greatest source of harmony at home.

Manners are also easy to teach to the young, for little children are like monkeys. You and your husband just have to be polite to each other and to say "please" when you ask your son to do something, and "thank you" for doing it, and he will soon imitate you routinely. If you remind him, however, he'll object more and more because he has reached the age of independence. A child between 18 months and 3 years old is much like a young teenager. He's fun. He's silly. He's rambunctious. And he thinks he's a whole lot smarter than you are.

At this age he wants to decide what to eat, when to sleep and what to say. This is both natural and necessary. The more a child is allowed to rely on his own judgment, the more self-reliant he'll be.

This doesn't mean that you should be permissive about manners or anything else. You just need to guide your son more indirectly in these early years--and in early adolescence--so he will do what you want most of the time and even think it was his idea.

When you get to the door, for instance, wait until he tries to open it and then gently do it yourself, all the while thanking him for opening the door for you. He'll think he did it, and one day he will open doors for you without being asked.

You can teach him to help others by asking him which small bag he'd like to carry from the market. If you give your child as much responsibility as he can handle, he will not only become more competent and more confident, he will seek more responsibility, too.

You can also teach a few table manners to your son by the time he's 3, but not by correcting his every bite. Instead, take him aside before a special family dinner, review the rules quietly and then rely on an occasional raised eyebrow when he talks with food in his mouth or puts his elbow on the table. He'll eventually improve, if you don't raise your brow more than three to four times during a meal. But you can still expect these and all manners to fall apart many times between now and the day he leaves for college.

Sometimes children have relapses because good manners can get boring or because their friends don't have any or because it's harder to have good manners at certain ages. A 4-year-old often forgets his table manners because he's busy making faces out of the food on his plate; an 8-year-old, that silly soul, is trying to burp and say "Ralph" at the same time, and a child of 11, 13 or 15 may be too self-focused to be polite.

If you don't make a big deal about his lapses, your son will find his manners again and, in fact, he'll never even know he had lost them.

Please send your questions to Box 15310, Washington, D.C. 20003, or to margukelly@aol.com