THE ENGAGEMENT, that sweet time before an emotional commitment is turned into a legal commitment, is, of course, a trial period.

And the reason it is such a trial is that a couple who can get through an engagement, with all its etiquette land mines, ought to find marriage a cinch. The failure of so many young marriages, however, leads Miss Manners to suspect that these people are not enduring sufficiently rigorous engagements, and are doing their fighting after marriage, when it can take a nastier turn.

The popular belief that sex and money are the two subjects that couples split over is erroneous. People who love each other don't break up over such trifles. But they must learn to handle the explosive subject of conflicting etiquette. For example, a situation where one marriage partner is feeling playfully amorous and the other is taking the bar examination in the morning is an etiquette problem of dangerous proportions.

Here, then, is a check-list of etiquette fights an engaged couple should have, so as to be ready and tried enough for a peaceful marriage.

Miss Manners is presuming that the couple is well-suited and in love, and that fortune and their families are smiling upon them. Six Months Before

Fights over whether wedding is to be formal or informal.

Fight over what is meant by formal and informal.

Fight over size of wedding.

Fight over whether 75 people can be considered a small wedding.

Fight over whether silver and china fit into your way of life.

Fight over whether silver and china are a better investment than stereophonic equipment. Three Months Before

Fight over whether anybody can really tell the difference between engraving and raised print.

Fight over whether relatives who had bitter divorces should be invited, as well as their former spouses.

Fight over whether friendship means more in selecting wedding attendents than auditioning for physical types to present a harmonious, chorusline background.

Fight over whether wearing a white wedding dress will be worth the sneers of people who believe this must symbolize an unopened package.

Fight over whether the mother of the bridegroom should be forced to wear a type of dress she dislikes in order to be visually paired with the mother of the bride who finds that style flattering. Two Months Before

Fight over the discovery that the bridegrooms's family has not only exceeded its quota of guests, but has provided a list using initials instead of names and terms such as "and family" for children.

Fight over whether guests' requests to bring their current love interests should be honored and who is going to tell people that their small children will not be welcome.

Fight over slurs made on relatives who sent cheap or tasteless presents. One Month Before

Fight over whether it is the bride's or the bridegroom's mother who is at fault because elderly friends are beginning to complain that their presents have not yet been acknowledged.

Fight over whether the seating arrangments should be done according to tradition or according to who is speaking to whom. One Week Before

Fight over the failure of some guests from each side to answer invitations and about who is going to prod them so as to provide an accurate number to the caterer.

Fight over how much luggage is to be taken on the wedding trip.

Fight over the wisdom of marrying a person now discovered too short-tempered, stylistically alien to one's tradition and completely absorbed in petty details to the exclusion of any intellectual or romantic activity. At the Wedding

Fight over whether the ceremonial kiss was intended to demonstrate enthusiasm for the marriage or to protect the bridal makeup.

Fight over whether it is each other, the wedding guests or the photographer who deserve the bridal couple's chief attention during the reception.

This is only a minimal list, and every young couple should feel free to make additions which express their personal relationship. For example, there are people to whom a wedding reception means dainty tea sandwiches and champagne only, and others for whom no wedding is recognized by God unless everybody present is groaning audibly and able, through the bounty of the hosts, to go home sick. If the mothers' dresses fight appeals to you, you may expand it by attempting to force all grandmothers to match, which could stir up a great deal of irascible excitement.

The important thing to remember is that no one rule of etiquette is as essential as the general atmosphere of conflicting standards. People who have conscientiously fought out all such matters during engagements will find themselves only too grateful to be living happily ever after. MISS MANNERS RESPONDS

Q: It is a matter of pride to me that my food looks as good as it tastes, and I certainly enjoy being told that an aspic or, especially, one of my fancy desserts looks spectacular.

But the way people put it usually is, "Oh, I'm not going to spoil this -- it looks too good." I know they mean well, but I'm standing there holding a platter out to them, and they don't do anything until I've said "Go ahead" over and over until I lose my patience.

It's heavy, after all, and I have to go around the table with it. How do I convey the idea to the first person that I made it to be eaten?

A: By saying, "Well I hope you'll change your mind," and moving on to the next person. By the time you get back to the first person after everyone else has been served, the problem -- if not the dessert -- will have disappeared.

Q: The other night at the dinner table, my 8-year-old son started to eat his asparagus with his fingers. When I brought this to his attention, my wife informed me that it is considered good manners to eat this with the fingers. Good etiquette tells me you don't use the fingers for this vegetable. Your opinion please?

A: But Miss Manners tells you that you do. Asparagus is, indeed, correctly eaten from the fingers, in a very old tradition of which few modern people seem aware. Those who do know can therefore have a marvelous time doing this in company or in restaurants and being reprimanded or at least stared at, only to have the disapproving people find out later that they were in the wrong. What Miss Manners wonders is how an 8-year-old boy found out this? Would he like to meet a refined Victorian lady?

Q: We recently attended a wedding where the groom's step-mother and bride's step-grandmother were not given corsages. To me this looked bad. What is your opinion?

A: The number of people who are expected to be distinguished with floral arrangements at a wedding has multiplied over the years at the instigation of the flower industry, no doubt, to include mothers and grandmothers of the principals, as well as their sttendants. However, the number of mothers and grandmothers per bride or bridegroom also has multiplied considerably. Miss Manners' opinion is that is it silly to have one bridal couple with what look like four mothers and eight grandmothers. Limiting these decorations to the original relatives -- or eliminating them (the flowers) altogether -- seems more sensible.

Q: What is the proper etiquette on a crowded bus? If some little old lady with a pointy umbrella hits me in the shins first, may I kick back?

A: It is rare, nowadays, to have a deep-seated, well-motivated sensual urge that you are forbidden ever to gratify. However, Miss Manners must tell you that kicking little old ladies in the shins is one of them. (Scream "ow!" and stare at her with a frightened expression. She will take herself and her umbrella to another part of the bus in a hurry, thus solving your immediate problem if not satisfying your heart.)