A 1977 Valentine sentiment: If you think YOU'RE lonely, try to put yourself in my place . . . say about 7 o'clock? HAPPY VALENTINE'S DAY.

So far, Hallmark, hasn't produced Valentines saying explicity: To the One I Live With Without Benefit of Matrimony, or, To the One I Love Despite the Fact that We Are of the Same Sex. On the other hand, Donald J. Hall, president of Hallmark Cards Inc., is convinced at least one of his cards would do in each of these cases.

Hall is pretty safe in his conviction - every year he approves 18,000-odd greeting card designs, produced by his 300 artists, seven writers and seven editors in Crown Center in Kansas City, Mo.

The Hallmark home - a Middle American rival to Rockefeller Center - covers 25 square blocks and will eventually include 50 buildings, including offices and residences.Hall talked about his life as king of the cards when he was in Washington recently to watch as his father, Joyce Clyde Hall, the 86-year-old founder of the firm, was induced into the Business Hall of Fame.

The biggest trend in greeting cards of all sorts, according to Hall, is toward individuality . "We offer mix-and-match choices of sentiments, and the black card for writing your own message is way and above the most popular. There is a tremendous demand for diversity. It's no longer a case of all nostalgia or all art cards or all nature subjects. People today want to pick and choose to suit themselves. You can't tell them. This is the fad this year," he said.

"I think people are more confident. They don't need someone else to express their feelings for them. And even when they do select a card with a sentiment, they like it to be short, an they can add their own message. (I just to think of you as my Valentine/As a matter of fact, I just like to think of you. Happy Valentine's Day. ) With Christmas cards, we're seeing a great decline in the printed signature."

This time of the year is almost as sweet for Hall as his Merry Christmases, because "Valentines are our second-biggest sellers, right after Christmas cards," he said. "Mother's Day and Easter are tied for third place."

Love, at least as marked in the sending of valentines, seems to be on the upswing, Hall says. "For the last year or two we've been seeing a trend to sweeter sayings, a swing away from the more militant '60s sentiments. (No pedestal for you. No pedastal for me. That's what I like about us.) The pictures are softer, as well. Especially the young people seem to select the tender, loving cards. There's a great deal of poetry being written and read today. By men, too."

(Today's the day for Valentines the special day each year/We set aside to let friends know that they are very dear. And so it seems the perfect day to greet a friend like you/Who's thought so very much of today and all year through.)

This softening trend, Hall says, also means almost the total disappearance of the penny dreadful: the vicious valentine. "Of course the comic valentine is still with us. (Valentine, you are a passionate creature! You are consumed with mad desire! You cannot control yourself! - When I snap my fingers you will awaken and remember nothing.) .

Some verities are still with us though: The penny and nickel valentine are still produced as well as the package of assorted valentines for classrooms. Hall believes that children send more valentines than anybody. And the favorite recipients are Mother and Teacher. The make-your-own valentine kit, with hearts and lace to cut out, paste and color, are still great sellers.

The only sour note for Hall this year is that Valentine's Day is on a Monday. "Valentines are a last-minute thing. Most people go out and buy their valentines in the last five days before the biggest day of all. It hurts us when Valentine's eve is on a Sunday and a great many of our outlets are closed."

The cinnamon heart-red color traditional to valentines is not only with us this holiday, but strong colors are coming back for all greeting cards, according to Hall. "We're already well into our 1978 Christmas cards. So I can say with some assurance that the softer, natural colors are declining in favor of a strong swing back to dominant colors."

What Hallmark calls the "contempory card" a separate division in Hallmark, is still going strong, despite the trend toward sweetness and light. These are the cards with the supergraphic designs and the current causes, like the Santa Claus wearing the gas mask. But Hall says that like nostalgia, they're just one more into the pot.

Even with all those 18,000 designs, Hall says there is no shortage of ideas. "We watch the posters, the billboards, music, art. For instance, during the pop art period, we did a series based on labels, like the Green Giant Gosh was that tricky legally.

"The sentiment is always first. The illustration follows. Because we know people may be attracted to a card by its visual appeal, but they won't buy it if it doesn't read well."

Hall, sadly, deflates one of the great get-rich-quick dreams. "I can never remember us ever buying a sentiment from anyone who didn't already work for us. We get 50 ideas in the mail a day, and we mail them all back to people . . . We do buy a few ideas for our contemporary cards, but only from people who have worked for us before. Then we only pay anout $50 each."

Hall, who inherited the presidency of the company from his father in 1966, is a quiet man who looks like the 50-year-old Kansas City businessman he is. He graduated from Dartmouth College in Hanover, N.H. - while managing a greeting card sales territory nearby.

Despite his business orientation - he majored in economics - he has a fondness for the design part of the business. His own contemporary home, for his wife and three children, is designed by a Kansas City architect. For Crown Center, he and his father picked the well-known New York architect, Edward Larrabee Barnes, for the master plan as well as the first units: the retail complex, office buildings and bank. Architects Collaborative, the old Walter Gropius firm, designed the new residential complex with 133 condominiums in a skyscraper tower, an innovation for that area.

And what kind of a Christmas card does Hall himself send? "The card which is also a tree ornament. You know people are collecting them now. I heard the other day of one just two or three years old which is selling for $30."