What was going on when I was born?

The triumphant researchers who found the luxury liner Titanic were greeted at their home port here today by cheering crowds, balloons and the blare of airhorns. Two helicopters and a flotilla of little boats followed the 245-foot Navy research vessel Knorr to her landing as the exhausted discoverers waved to families, friends and hundreds of reporters from around the world. ''I'm glad it's over,'' Dr. Robert D. Ballard, leader of the research team and a senior scientist at the Woods Hole Oceanographic Institution, shouted from the deck as the Knorr pulled up to the institution's wharf. The first man to shake his hand was Rear Adm. J. B. Mooney Jr., Chief of Naval Research. The Navy financed the development of the underwater robot sled Argo, which last week took more than 12,000 photos of the liner, which sank 73 years ago with the loss of more than a thousand lives. The Navy wants to use the Argo for top-secret military missions, according to naval sources and scientists who work for the Government.

Federal regulators refused today to block the trucking of spent nuclear fuel through New York City. The regulators, from the Department of Transportation, acted in a case brought by New York City, which claimed that the city's dense population made it unsafe to move the radioactive material through Queens and the Bronx. Officials in Connecticut were jubilant over the decision because they had feared that the nuclear material would be detoured through their state if New York City had won its case. ''We've taken a little bite out of the Big Apple,'' Joseph I. Lieberman, the Connecticut Attorney General, told a news conference in Hartford.

''Our influence over South African society is limited. But we do have some influence, and the question is, how to use it.''

EARLY tomorrow, as the climax of an extraordinarily audacious and complex series of space acrobatics, a small spacecraft will plunge through the tail of the comet Giacobini-Zinner. The mission, by the International Cometary Explorer, will be the first to a comet and will provide a dress rehearsal for the multiple fly-bys of Halley's comet next March. During the precious one or two hours of its passage through the tail, radio observatories from Japan to Spain will seek to record transmissions from the spacecraft as it makes the first direct observations of the comet's composition, magnetism and other properties. Spacecraft have visited the Moon, analyzed the soil of Mars, passed close to the great outer planets, their moons and rings. Meteorites of diverse types are in our museums. But until now we have never had a close look at a comet.

Even though car and truck sales will hit record levels for the 1985 model year, the chairman of General Motors Corporation warned today that the American auto industry, and others as well, would have to move swiftly to computerize their operations to meet threats from abroad. Roger B. Smith predicted that total American sales of cars and trucks -domestically made or imported -would reach 15.4 million by the end of the month, which is the end of the 1985 model year. The previous record was 15.3 million in the boom year of 1978. And he said he expected 1986 to be strong as well, though perhaps not reaching a record. ''We have a real shot at surpassing 15 million units again,'' he said at a news conference.

SHARP disagreement has developed between school reformers and academic researchers over the emotional issue of whether to end ''social promotion'' in public schools. In an effort to shore up academic standards, many local school districts have adopted policies denying promotion to students who fail to meet certain standards in mathematics and reading. But researchers say that, in practice, such policies are ineffective and even counterproductive. ''You have two competing elements,'' said Nathan Quinones, the New York City schools chancellor. ''The reform movement, which is relatively conservative, wants to emphasize standards and accountability. But most research shows that holding students back doesn't help them academically.''

Top executives at the RCA Corporation and MCA Inc. failed to reach an agreement about a possible merger this weekend, and no further talks are scheduled, according to a source close to the negotiations. An MCA executive declined to comment, and RCA executives could not be reached for comment on the talks. Partially as a result of the failure to reach an agreement, RCA said yesterday that its board had approved a new ''poison pill'' provision that would make a hostile takeover difficult, even though there has been no indication of a pending bid for RCA. The source, who asked not to be named, said that RCA's adoption of the poison pill package was partly a result of the failure of the two companies to reach an agreement. An acquisition of MCA would have made a takeover of RCA more difficult, because it could have raised the cost of acquiring RCA for any hostile bidder.

The Paradyne Corporation and the Securities and Exchange Commission announced today that they had reached a settlement that avoids trial of a civil complaint charging that Paradyne had violated Federal securities laws to obtain a $100 million computer contract from the Government. In the settlement, the company, based in Largo, Fla., did not admit or deny wrongdoing. The settlement was approved just two days before the trial was scheduled to begin before Federal District Judge William Terrell Hodges in Tampa, Fla.

Mayor Koch and his two major rivals sought votes on the streets and in the subways yesterday in a final spurt of campaigning before today's Democratic primary. Seeking out the voters at the evening rush hour, a confident Mr. Koch pushed head-on into homegoing crowds of pedestrians in midtown. One of his opponents in the mayoral race, City Council President Carol Bellamy, campaigned among subway riders at a station in Forest Hills, Queens, while another, Assemblyman Herman D. Farrell Jr., concentrated on a subway stop in his Washington Heights district in Manhattan. Earlier, the three made an unusual nondebate appearance together on the City Hall steps to encourage a large voter turnout today - encouragement that was needed in view of predictions that only about 30 percent of the 2.1 million Democrats in the city would vote.

International The President, in a major reversal of the Administration's opposition to economic sanctions against South Africa, announced he was adopting most of the trade and financial actions against apartheid that are sought by Congress. Mr. Reagan barred most loans to Pretoria and the sale of computers to its security agencies. [Page A1, Columns 4-6.] South Africa denounced President Reagan's announcement of sanctions and said ''outside attempts to interfere'' in Pretoria's affairs could only retard racial change. [A12:1-2.]

RESEARCHERS charting the course of love are beginning to put some order into an area that has long been regarded as chaotic and undefinable. The emotional turbulence of love and marriage, the research suggests, can be traced partly to the way habits of the heart are shaped in childhood, and partly to the inevitable changes over the course of even the most successful relationship. ''People don't know what they are in for when they fall in love,'' said Robert Sternberg, a psychologist at Yale University who is one of those doing the new research. ''The divorce rate is so high not because people make foolish choices, but because they are drawn together for reasons that matter less as time goes on.'' What brings a couple together, Dr. Sternberg is finding, almost inevitably recedes into the background as the relationship matures. And those qualities that matter most later on, his research suggests, are rarely the ones that loom large in the early stages of the match. In his view, the major components of love are intimacy, passion and commitment. While a relationship can manage to survive with any one or two of these qualities, Dr. Sternberg argues, the fullest love requires all three. However, each blossoms at its own pace, and follows its distinct course.

An attempted coup led by former Thai Army officers was thwarted today after Government troops forced the rebels to surrender after less than 10 hours of fighting. At least four people were killed in artillery and automatic weapons fire, and 60 people, at least 29 of them soldiers, were reported wounded. Two of those killed were members of an NBC News television crew who were slain during a battle at an army compound shortly after the rebels proclaimed the coup early this morning. A soldier also died, and a civilian Thai riding in a taxi was fatally wounded when a shell landed in traffic in central Bangkok. #400 to 500 Soldiers Involved Officials here estimated that 400 to 500 soldiers and officers were involved in the uprising, and that most of them had surrendered by midafternoon. Most were said to have returned to their barracks.