YOU THINK that married people have problems? You think that single people have problems? What about people who aren't either?

Miss Manners is often asked what is the major etiquette problem of our day. It is: What do you call the person with whom you live (or your child lives) to whom you are not married?

And she regrets to say that she hasn't solved it, either. The need is for a word that is not condescending (mistress), childish (boy friend, girl friend), cute (lover) or confusing (roommate). Reader suggestions have included consort (which make Miss Manners think of Prince Philip walking three paces behind) and covivant (which sounds to her like someone who will only cook on copper pots).

Miss Manners once proposed "partner," as in marriage partner, but without the marriage. It was not received with cries of delight by a grateful nation, and Miss Manners, feeling miffed, took to asking querulously why people needed to declare their sexual affiliations, anyway.

They never used to, she would point out peevishly. There is nothing new about sex, many of our ancestors have practiced it, and there were always people who followed the emotions rather than the law. They got on perfectly well with only three categories of relationships that were publicly recognized or admitted: marriage, engagement and just-good-friends. That last one was an excellent catch-all, and Miss Manners would ask why it couldn't still be made to do.

But the truth is that it can't.Like it or not, there is hardly anyone who does not know people who reject legal sanction for their union but demand social sanction. Now that married people recognize that they can be happy without acting as if they were in a potato sack race, unmarried couples tend to be as finicky about being invited, introduced and otherwise treated as a unit as 1950s teen-aged newlyweds.

This gave Miss Manners the clue that she had, when looking to the past for precedent, looked too far. The 1950s had a rigid protocol for defining premartial affilations among the young: going in the same crowd, double dating, dating, going together, going together steadily, going steady, being pinned, being engaged to become engaged, and being engaged. (There were geographical restrictions on each, but that's another story. The point here is that, like Eskimos with many different words for snow, they recognized distinctions and gradations not perceptible to outsiders.)

Let us now go to work on the problem of the 1980s. We need a vocabulary and code of behavior for the unmarried couple. These must be sensible; simple and dignified, because they are for public, daylight use. "The woman (or man) I share my life with," for example, is none of the above. Imagine that as an introduction of a legally married spouse, and your stomach will show you what Miss Manners means.

She is still attached to the word partner, and tries to remember that there are still women who say they hate the term "Ms." and then explain why neither Miss nor Mrs. is right for them, either. When a solution is offered, it is greedy to push it away and ask for another.

But Miss Manners will also propose the old term "trial marriage" to describe these unions, and a warily put forth "trial husband" and "trial wife." dNo, not really. What she will put forth is the request that we all keep trying. MISS MANNERS RESPONDS

Q. A friend and contemporary invited me and my wife to bring our granddaughter to her country home for dinner. Granddaughter and this friend are alumnae of a pretigious Eastern college. There are 50 years between the dates of their graduation.

A year later, I arranged a luncheon party in a restaurant for the four of us. Granddaughter is attending graduate school in our city. The evening before the luncheon appointment, granddaughter telephoned: A young woman from her home town had come unannounced to visit her. "Would it be all right if she were to bring this friend to lunch?"

I was confused.I felt that the two alumnae should not have their mutual conversational interest diverted. I offered the opinion that under the circumstances, we ought not to invite the young lady. (There was no problem in providing her with lunch in my granddaughter's apartment.) What is your opinion about such a situation? Who should have been favored, the older alumna or the unannounced visitor?

A. As you will know, age, both of the alumna and of the luncheon engagement (as opposed to the age of your granddaughter's commitment to an unannounced visitor) take precedence here.

But while there was nothing wrong with your decision. Miss Manners questions whether it was, in fact, in the best interests of your contemporary. The luncheon was not an alumnae meeting -- you and perhaps also your wife did not attend the college -- nor a first meeting for the purpose of discussing the college. As a return invitation, it suggests that your friend and your granddaughter enjoyed meeting each other the first time. Perhaps the elder lady might have enjoyed widening her acquaintance still further by meeting her new friend's friend, which is always a flattering request.

Q. I have two burning questions that I wish you would respond to. The first is, what do you do with a Miss Manners who does not respond to her mail? The second is, again, what would you, with your 19th-century rules, call Mrs Sr. when there is no longer a Mr. Sr. and everyone else has moved up in the male line by your rules . . . Mrs. Left-Over?

In today's world there are unnumerable legal documents made out in "Jr.," "III," etc., and there would be much effort and years of confusion in changing them, not to say anything about daily mail. They include deeds, titles, licenses, bank accounts, bonds, telephone directories, wills and so on. Mail sometimes comes to this house addressed to "William Leavitt Sullivan," and I never know whether to mark it "deceased, never lived here, return to sender," open it -- which would be illegal if not intended for me -- or refer it to my son, who would open it illegally if not intended for him. Thus, the "Jr.," "III," etc., play both a legal and continuing role, in spite of Miss Manners.

Your comments about the card of "Mrs. Sr." prove that you are way off base, and ought to stop mis-leading people.

A. Miss Manner is deeply wounded by your only-too-true observation that she is desperately behind in the struggle to keep up with her mail. But not deeply enough to give up misleading people.

There are two ways of asking, and of answering etiquette questions. One is "What is correct?" and the other is "Do you think it's all right to do what suits me best anyway?"

Correctness refers to whenever a complete set of arbitrary rules about civilized behavior was last codified. In the case of Anglo-American society, 19th century is about right. Aren't you lucky that we're not still using the rules from the Court at Urbino?

By this strict usage, the oldest living male of a name does not use "Sr." or any other suffix; "Mrs. Sr." is used by a widow to distinguish herself from the wife of the man who is now the oldest person of the name. "Jr." is not part of your name, Sir (Miss Manners almost addressed you as "Junior"). There also are legal documents with "Mrs." or "Dr." used as if they were part of names, but they aren't. If legal and correct usage were identical, lawyers would be the best behaved people on earth.

However, while Miss Manners has a head full of these nasty little strict rules, which she is delighted to supply on request, she sees no reason that people who do not wish to follow them should have to do so. If a lawyer is not to be confused with an arbiter of etiquette, the latter should not be confused with a traffic policeman.

Q. What could be done with holiday and birthday gifts that are of no value or use to the receiver? I have in possession several gifts that I can't use, and would hate to return to the giver for inconvenience. Several years ago, I read that it's only proper to give unworthy gifts to charity for resale, but with garage sales so popular, I'd give it a try.

A. Properly speaking, there is no such thing as an unworthy present, so you may dismiss what you read about disposing of them.

The minimum use that a present has, no matter how dreadful an object it is, is to please the giver with the mistaken notion that you appreciate it. If you are clever, you may keep this misconception alive through your thanks, while quietly ridding yourself of the offensive object by returning it, selling it or giving it away. The only thing to watch out for while doing any of these things is that the person who gave the present is unaware of this transaction. Do not have him at your garage sale.