DEAR ABBY: My mother-in-law is 84 years young. She is dating a 58-year-old man. They met 18 months ago and see each other every night. On weekends he picks her up and they go to his place from Friday until late Sunday evening. This man says they are compatible.

"How can this be?" my husband and I ask. At times she doesn't even remember being with him the entire day before.

Abby, this man is younger than her four sons. Each of them has tried to talk to their mother about this relationship. She sometimes "hints" that the relationship is sexual, making comments like, "Well, at least I don't have to worry about getting pregnant."

She can't be talked to or reasoned with, because she starts screaming that her family is "jealous" and trying to "break them up."

We're at our wit's end and don't know where to turn. She has money. He has none. Thank you from the bottom of our hearts if you can offer any advice. -- READY FOR THE BOOBY HATCH, JACKSONVILLE, FLA.

DEAR READY FOR THE BOOBY HATCH: If your 84-year-old mother-in-law's memory is impaired to the point that she doesn't remember having spent the previous day with her gentleman friend, one wonders how she is handling the rest of her affairs. Is this the way she has always behaved?

If the answer is no, she may be due for an evaluation by her physician to determine if she is still capable of handling her financial affairs. If a medical examination confirms that her memory lapses are severe, the family lawyer should be consulted to ensure that she cannot be taken advantage of financially.

Once that precaution has been taken, I see no reason why your mother-in-law shouldn't continue a relationship that both she and her friend enjoy.

DEAR ABBY: I have planned my last party. Out of 45 invitations mailed out, we had three RSVPs! The invitations clearly stated a date by which we needed a response, and still only a tiny percentage complied.

My husband and I would not have been as upset if those 42 people had called with their regrets. But sitting around and wondering how many, if any, people are going to show up is nerve-wracking. I had no idea how many people to prepare for, or whether we needed to rent tables, chairs, etc.

The same thing happened with our wedding invitations. They included a response card and a self-addressed, stamped envelope (all they had to do was indicate whether they were coming or not, and put it in the mail), but many people never responded to the invitation. Some showed up without letting me know they were coming, and others failed to show when they said they would.

Has something changed? Why do people think it's OK not to respond to an invitation when specifically asked to do so? -- FED UP IN MILWAUKEE

DEAR FED UP: Nothing has changed. "R.S.V.P." stands for "repondez s'il vous plait," and translates to "please respond." To ignore an invitation, and neither accept it nor politely refuse, is inexcusable. It takes only a moment to make a call or return a written reply. However, if people haven't responded in a reasonable period of time, I see nothing wrong with calling them and asking if their decision is "yea" or "nay." As a host or hostess, you need to know how many guests to prepare for.

Everybody has a problem. What's yours? Get it off your chest by writing to Dear Abby, P.O. Box 69440, Los Angeles, Calif. 90069. For a personal reply, please enclose a stamped, self-addressed envelope.

DEAR ABBY: My youngest daughter, "Martha," is 18 and has moved back home after two months on her own. I never knew how much I valued my privacy until my husband and I had a chance to spend time alone. Now my life has been turned upside down.

Don't misunderstand me. I love Martha with all my heart and would be devastated if any harm came to her. But she uses my perfume every day because she doesn't want to buy her own and "borrows" my clothes at will. Every night I have to retrieve my belongings from her room.

If I talk to my husband about our daughter, he defends her. If I confront Martha, she runs to Daddy. The tension around here is causing friction between my husband and me.

The other night, Martha came home at 1:30 a.m. and proceeded to fix herself a snack in the kitchen, waking me up. The next morning at 5:30 a.m., I turned on the stereo, turned on the lights and made so much noise that nobody in the house could sleep. My husband thinks I have lost my mind, but I'm determined that my daughter is going to have a dose of her own medicine.

This girl pays no rent and brings home $200 a week. The only thing we ask of her in the way of housework is that she keep her bathroom clean. We insist that she contribute $25 per week for food and purchase all of her personal items. So far, she has paid on time, but she is starting to run out of supplies in her bathroom, and I'm sure they'll appear on our grocery list on Friday.

How am I going to keep peace in my home? Am I being petty, as my husband says? This could destroy my marriage if it continues. Please help me. -- HELL ON THE HOME FRONT IN FLORIDA

DEAR HELL ON THE HOME FRONT: Since your daughter was away from home for only two months before returning, I have to wonder how you coped before she left. However, since she is employed, you are doing the right thing to charge her for food and insist that she purchase her own personal items. It will help ease her into the financial realities of being on her own.

Difficult economic conditions are keeping many adult children in their parents' home. But remember, it is your home. A grown child lives with you by privilege, not by right.

If the friction continues, you and your husband might consider subsidizing her in an apartment of her own. The continued success of your marriage could depend on it.

DEAR ABBY: I just finished reading the letter from "Alone in Ann Arbor, Mich.," who wants to save sex until after marriage, but was being pressured by men who wouldn't see her anymore unless she had sex with them. They wanted to take a "test drive" before they made a bigger commitment.

Well, I am a 27-year-old male virgin and I don't understand why more people can't grasp the logic that sex before marriage is not a good idea, not just because of AIDS or questions of morality, but because of wanting to have a relationship that will last in the long run.

No lasting relationship can be based on how good the sex is, because one day the sex is going to slow down or even run out. What you're left with after that had better be pretty solid, or you may as well start shopping for a divorce lawyer while you're shopping for the ring. I know that if I can say I want to marry someone and, in effect, spend the rest of my life with her without ever having had sex with her, the sex is only going to make that relationship better.

Has no one else seen the correlation between the increase in premarital sex and the increase of divorce in the past 20 years? I'd bet my life that there's a direct relationship between the two. It seems to me that the problem is that people think love should be based on sex, when it's actually the other way around.

I congratulate "Alone in Ann Arbor" and every other person who, for whatever reason, has the determination and wisdom to abstain from sex until they're married. -- NO NAME, GREENVILLE, S.C.

DEAR NO NAME: And I congratulate you on your clear-headed thinking.

Good advice for everyone -- teens to seniors -- is in "The Anger in All of Us and How to Deal With It." To order, send a business-sized, self-addressed envelope, plus check or money order for $3.95 ($4.50 in Canada) to: Dear Abby, Anger Booklet, P.O. Box 447, Mount Morris, Ill. 61054-0447. (Postage is included.)

DEAR ABBY: After reading the letter from "Roger From Walnut Creek, Calif.," I was so furious I had to respond.

In Roger's opinion, "99 percent" of female runners wear sports attire that leaves little to the imagination in the hopes of attracting the attention of men.

I am a female runner who lives in Texas where the summers are extremely hot and humid. Running with heavy clothing is not only uncomfortable, it's also dangerous because there is no chance for the skin to breathe. I run anywhere from 4 to 6 miles, and in the summer this is often sheer torture. However, I and other female athletes continue to train in order to maintain health and fitness.

To suggest that we run in skimpy attire because we are seeking attention from men is just plain ignorant. There are a number of ways to get attention from men without putting oneself through a grueling workout in suffocating heat. Women receive catcalls and wolf whistles whether they wear shorts or baggy sweats. Most women find this "attention" childish, immature and often downright scary.

I feel sure Roger is not a runner. If I am wrong, I'd like to invite him to come to Texas next July and run 5 miles with me. I will wear my so-called skimpy outfit and he can wear his sweats. We'll see who collapses first. My guess is it will be Mr. Big Mouth. -- THERESA SIBLEY, FORT WORTH, TEXAS

DEAR THERESA: Roger's letter rankled other female runners, who were quick to point out that their sportswear can affect the quality of their performance. Read on:

DEAR ABBY: I'm responding to "Roger From Walnut Creek, Calif.," who thinks women runners wear tight clothes so that men will look at them.

Abby, Roger needs therapy! I run every day with several other women, and we wear EXERCISE CLOTHING. The form-fitting lycra/spandex material allows for greater range of motion and the breathable fabric keeps the athlete drier, which adds to comfort. Greater comfort, better performance. What we are focused on is comfort and performance, not attracting attention from men.

Please tell Roger that women have better things to do than collecting catcalls from strangers. Maybe he should get out of his easy chair and try a little exercise himself. On second thought, we're better off if guys like Roger stay inside! -- JUDY B., PORTLAND, ORE.

DEAR ABBY: This is also in response to "R.H.G. in Elm Grove, Wis.," who wrote to say that her mother would not wear hearing aids because she thought they would make her look old.

She has things turned around. It's being hard of hearing that makes people think you're old. Don't fool yourself; they will figure out quickly that you are hard of hearing, or worse yet, they'll think you're senile because you respond incorrectly to questions.

Since I often didn't answer questions or enter conversations, many thought I was a snob. The truth was I hadn't heard them.

Thirty years ago when I got my first hearing aids, they changed my life. My whole world became a beautiful place full of sounds. I laughed and talked more -- and I became an active participant in what was going on around me.

So, to your readers I say: Hurry to your phone and call for an appointment with an otolaryngologist. (That's a doctor who specializes in ear, nose and throat problems.) You will never be sorry. -- AN 83-YEAR-OLD GREAT-GRANDMOTHER WHO IS GLAD TO BE HEARING HER GREAT-GRANDCHILDREN, INDIO, CALIF.

DEAR GREAT-GRANDMOTHER: Now that people are living longer and enjoying their golden years more, your advice should be sweet music to many.

To order "How to Write Letters for All Occasions," send a business-sized, self-addressed envelope, plus check or money order for $3.95 ($4.50 in Canada) to: Dear Abby, Letter Booklet, P.O. Box 447, Mount Morris, Ill. 61054-0447. (Postage is included.)