THE chief duty of the bridegroom's family is to pretend to be crazy about the bride some fine this as diffecult a task to sustain as all the planning, financing and execution of the brides's family's duties put together.

Nevertheless, it must be borne constantly in mind, as the bridegroom's parents are run through a routine not of their own making. They have few tasks, but constantly repeating how lovely everything is--the bride's grandmother, the wedding silver, the bridesmaids' dresses, the striped tent on the lawn, the wedding breakfast, the promises about when thank-you letters will be written--is essential.

Miss Manners sympathizes with those who thus find themselves compelled to endorse things they abhor--raised-print invitations, eccentric clothes calling themselves formal, and so on. But this is not an occasion on which the most nearly perfect form of etiquette predominates, unfortunately. The truly correct thing is to go along as much as possible with the decrees of the bridal family, and hope that a lifetime's instilling of values in the bridegroom will enable him to reeducate the bride as soon as the first blinding raptures have subsided.

The first of these jobs is to tell the bride, or to write to her, of her fiance''s parents' overwhelming happiness at welcoming her into the family. From the moment of being informed of the engagement, they are barred from discussing, in front of their son, whether her hair is dyed and why she talks so funny.

They must then call on her parents. "Call on," in these days when one's children marry outside of their parents' circles, can be a telephone call or a letter to another city, although for engagements of more than a weekend, it is polite to plan a trip to see what the young man is getting himself into. If the young lady has more than one set of parents, as also happens these days, the ones with whom she is or was resident are the most important, but it is proper to visit nonresident parents as well.

All the while, the bridegroom's parents should be madly declaring how lovely they find everything and everybody.

They may, if they wish, give a party to introduce the bride to their friends, but no parties--repeat, none--are obligatory on the part of the gentleman's family.

The next job is to supply full and correct names and addresses to fill that part of the guest list that is allotted to them. Meanwhile, they should keep saying how lovely they think the choice of wedding size and all other arrangements to be.

Miss Manners thinks it barbaric for a bride to attempt to costume anyone not in the wedding party. But the mother of the bridegroom, if told to wear her dress short or long, pink or gray, to match or complement that of the bride's mother, should remember what she thinks of the bride's and her mother's taste: She thinks it perfectly lovely. So are the flowers that they send her as a badge.

It has become increasingly common for the bridegroom's parents to give a rehearsal dinner the night before the wedding, but it is not traditionally obligatory. Neither is paying for any part of the wedding. Generosity is nice, but may be resisted, if demanded, on historical grounds.

They are expected to go to their allotted places at the ceremony and the reception, murmuring about how lovely it all is.

Then, as a reward, they may go home, take off their shoes and say to each other, "Did you SEE those people?"Q.I wear fine clothing, and although I have good table manners and am careful, sometimes accidents do happen and I get food stains on my shirt or necktie that are impossible to remove.My mother maintains that it is acceptable to wear a napkin tucked into one's collar in all but the finest restaurants or at formal occasions. I assert that this isn't done by adults, except in the privacy of their own homes, perhaps at a picnic, or when eating lobster. When dining out, I purposely avoid ordering foods that are particularly potentially dangerous.

I wouldn't tuck a napkin under my chin at a fast-food restaurant, much less anywhere else. My mother wonders why I am so concerned about what other people think. What do you say?A.Not being concerned about what other people think is a poor blanket lesson for mothers to be teaching is what Miss Manners thinks.The correct version is: In moral matters, you need not concern yourself with the opinions of others as long as what you are doing is both honest and kind. (Miss Manners brings you this one directly from her own dear mother.) In matters of etiquette, however, you do what is accepted unless, as in the case of civil disobedience, you wish to make a protest and accept the consequences.

Of course, adults do not wear bibs or make their napkins into them; nobody does who has passed the toddler stage. Life is not free of risk, but there are limits to how much one can protect oneself. Why doesn't she just serve you when you are in the bathtub?Q.I would like to know if it's necessary to write a thank-you note for presents my son receives at his birthday party. He will be 4. It seems ridiculous to me, but some of my friends do this.Miss Manners presumes you have taught this child to thank each guest with enthusiasm as the present is given. If so, you must be exhausted, and you may skip your written thanks and save any strength left for teaching the child to write thank-you letters when he becomes literate.

Copyright (c) United Feature Syndicate Inc.