What was going on when I was born?

The Ford Motor Company said today that it would buy 20 million of its common shares, continuing a stock repurchase program. It also said it was exploring ways to take more advantage of the value of its credit and aerospace subsidiaries, perhaps by spinning off and selling shares in the units. Company officials said the actions were being taken because of the low value the stock market has put on Ford's shares, despite its current prosperity. Ford earned a hefty $1.8 billion in the first three quarters of 1985, but its shares have been selling for about $48 each, which is barely more than four times annualized per share earnings.

Companies Beatrice accepted a $50-a-share takeover offer from Kohlberg, Kravis, Roberts. The $6.2 billion takeover would be the biggest ever outside the oil business. Kohlberg was chosen over a rival, a partnership of E. F. Hutton and Dart Group. To secure the deal, Kohlberg got an option to buy the grocery group and Tropicana subsidiaries for $2.39 billion, or the Tropicana, meat, soft drinks and bottled water businesses for $2.4 billion. [Page D1.] Donald P. Kelly, who is expected to lead Beatrice, is likely to sell off billions of dollars of operations, some analysts said. [D4.] Ford plans to buy 20 million common shares, continuing a stock repurchase program. It is also exploring ways to get more value out of its credit and aerospace subsidiaries. It might spin off the units, selling some shares in them but retaining control. [D1.]

''My mission, stated simply, is a mission for peace.'' - President Reagan. [A1:2.]

When South Korea's major shipbuilders announced layoffs of thousands of workers last week, they sent an alarming message to the other players in this industry: Even the world's most efficient shipyards are struggling. That the shipbuilding business, from Hamburg, West Germany, to Kobe, Japan, is hurting is no surprise. The industry has been reeling for years. What is new is that the problems have reached the South Koreans, whose low materials cost and inexpensive labor are the envy of the industry. ''You know things are in bad shape when the picture gets pretty bad for Japan and Korea, who together have about 70 percent of the worldwide market and are the price setters,'' said M. Lee Rice, president of the Shipbuilders Council of America.

The British and Irish Cabinets approved today a long-awaited agreement that is expected to give the Dublin Government a powerless but symbolically important advisory role in the affairs of Northern Ireland. Officials in Ireland and Britain declined to describe anything more than the broad outlines of the pact before Friday, when Prime Minister Margaret Thatcher and Garret FitzGerald, her Irish counterpart, are expected to meet to sign the accord. Treachery Is Charged But even without the details of the new arrangement, which follows a year of intensive negotiations, there were cries of ''treachery'' from spokesmen for the one million Protestants in the province. Throughout this century, the Protestants have clung fiercely to their British citizenship and resisted the nationalist idea of a united Ireland in which Roman Catholics would form an overwhelming majority.

The Beatrice Companies, owners of an array of brand-name products, including Samsonite luggage, Hunt's ketchup and Tropicana orange juice, yesterday accepted a $6.2 billion takeover bid from Kohlberg, Kravis, Roberts & Company, the nation's No. 1 organizer of management buyouts. The $50-a-share deal for Beatrice's 123 million shares, would, if completed, become the biggest takeover in the world outside the oil business. It would also become, by far, the biggest management buyout ever. On hearing the news, Daniel J. Good, executive vice president of E. F. Hutton & Company, who had organized the only competing bid, said, ''We wish Kohlberg, Kravis all the best.''

Friday 'CRUCIBLE,' THE OPERA This weekend and next, the Opera Ensemble of New York is performing a new production of ''The Crucible,'' Robert Ward's operatic version of the drama by Arthur Miller, which was based on the Salem witch trials. The cast is headed by Michael Willson, Adelle Nicholson, Deborah Lazenby and John Myers. The libretto is by Bernard Stambler, the production is staged by John J. D. Sheehan, and Michael Recchiuti will conduct. Performances will be given tonight at 8 and Sunday at 3 P.M. at the Lillie Blake School Theater, 45 East 81st Street. Tickets are $15. Information: 288-1485.

The catastrophic mud slides off a volcano in northern Colombia appear to be a classic example of what geologists call a lahar, a devastatingly fast and huge avalanche of mud. Scientists said the slides were probably touched off when heat from the eruption of the Nevada del Ruiz melted the covering of snow and ice. Lahars have been known to race down a mountainside at speeds as high as 60 miles an hour, sweeping away everything in their paths. Lahar, pronounced LAH-har, is a Javanese word. The island of Java, in Indonesia, witnessed some of the most destructive volcanic eruptions in history, including that of Krakatoa in 1883.

A dispatch from Los Angeles on Oct. 3 reported on California and Washington activities of Louis Farrakhan, leader of a Black Muslim group, and on plans for his Oct. 7 appearance at Madison Square Garden. As published, the dispatch noted that Mr. Farrakhan had called Judaism ''a gutter religion'' and had said Hitler was a ''great man''; it said these were ''statements that many consider anti-Semitic.''

A chart in Business Day yesterday with an article about the SIN Television Network incorrectly labeled figures representing the network's advertising revenues. They were in millions, not billions.

JAZZ fans know that the best way to hear jazz is to see it. The musicians may not emote like opera singers or leap around like rockers; instead, they react to one another's playing, listening and responding as fast as thought. On the bandstand, a cocked eyebrow, a brow furrowed in concentration or a quick grin can be a small epiphany. While jazz's audio legacy is available at any good record store, and current jazz can be seen nightly around the city, there is also a visual record of jazz - sketchier, and subject to the vagaries of producers and camera crews, but no less essential.

Culminating an emotion-charged test of the state's adoption law, New York's highest court ruled yesterday that the adoptive parents of a baby girl with Down's syndrome may keep the child, even though her biological parents now want her back. Calling the lawsuit involving two fit sets of parents ''tragic,'' the New York Court of Appeals unanimously reversed a lower-court decision and ordered that the 2-year-old girl, Marissa Stamatis, be left with her adoptive parents, Lisa Ann and Timmy Stamatis of Coram, L.I. ''Oh thank God, thank God,'' said Mr. Stamatis when he heard about the decision. The ruling was praised by adoption rights groups, who said it would strengthen the protection enjoyed by parents who adopt children. The Stamatises have raised the girl since November 1983, when she was 5 days old. The baby's biological parents, Christine and Warren Kosher of Syosset, L.I., had been seeking to reclaim her since she was 15 weeks old.