What was going on when I was born?

During Sunday's debate on the economy and domestic affairs, President Reagan played his strongest card by asking Americans whether they were better off than they were four years ago - a question he had used devastatingly in his campaign against President Carter. Whereas, four years ago, Mr. Reagan had suggested that the answer was no, today he thinks it is obviously yes. But Mr. Mondale, well prepared for the President's card, finessed it by pitching the issue to the future. ''The real question,'' he said, is: 'Will we be better off? Will our children be better off?' '' Mr. Mondale's questions focused on the future growth of the economy and what might arrest that growth unless the budget deficits are narrowed. He thus raised the question of whether Americans will be looking forward or backward when they go to the polls on Nov. 6. And, apart from the merits of the arguments, Mr. Mondale appeared to have scored with the public with his greater command of economic data and logic.

Political analysts and poll takers on both sides asserted today that Walter F. Mondale's performance in the debate Sunday night had created a chance for him to revive his campaign against President Reagan. But poll takers agreed that it would be several days before it became clear if the Democratic nominee got significant help in cutting Mr. Reagan's lead of 15 to 23 percentage points in most national surveys of voting preference. On the campaign trail today, Mr. Reagan's re-election team appeared on the defensive for the first time in weeks, as the President's spokesman sought to explain a series of polls taken Sunday night showing that many voters believed that Mr. Mondale had made a better showing in the 90-minute debate broadcast on radio and televison. Aides to the President, who appeared today in Charlotte, N.C., and Baltimore, acknowledged that the Mr. Reagan's performance had been less than overwhelming, but they said Mr. Mondale had not achieved ''a knockout.'' (Page A29.)

The Dining Out Guide in Weekend on Friday, Sept. 28, gave an incorrect address for the Parioli Romanissimo restaurant. It is at 24 East 81st Street.

IN an effort to learn, at last, the true dimensions of the universe, several groups of astronomers are undertaking highly automated searches of the heavens for exploding stars, or supernovas. Each group hopes to scan several thousand galaxies every night in the hope of glimpsing light that has not been seen there before, evidence of an explosion. Underlying these efforts is the belief that the light from exploding stars may give astronomers their long-sought ''standard candle,'' enabling them to estimate distances to galaxies at the fringes of the universe. A standard candle is a light source whose intrinsic brightness is well enough known so that the extent to which its brightness has been dimmed by distance can be used as a yardstick. When lights are observed across a darkened landscape their relative distances can be estimated only when one knows how bright they would be up close.

Police Commissioner Benjamin Ward announced new police communications procedures yesterday that direct the transit police to notify the city police immediately whenever a transit officer loses contact with a partner or is involved in police action outside the transit system. The directive, which was announced without comment at Police Headquarters, came in the aftermath of the killing of a transit police officer, Irma Lozada, on Sept. 21 in the Bushwick section of Brooklyn. She was fatally shot after she and her partner, Nat Giambalvo, split up to chase a suspected chain-snatcher who fled from a subway station.

Officials at the world steel industry's annual convention, which opened here today, gave guarded approval to President Reagan's program to limit steel imports. Steelmakers from Japan, Europe and Canada praised the plan, which would maintain their share of the American market. Several of them said that by curtailing the competition from the cheapest imports, the Reagan program might increase the prices they receive for the steel they sell in the United States. But steelmakers from such newly industrializing countries as Brazil, Spain and South Korea expressed anxiety about the Reagan program. Under the President's plan, the Administration aims to negotiate agreements that limit shipments from Spain, Brazil and South Korea, all of which have greatly increased their steel shipments to the United States in the last year.

The Challenger astronauts lost much of their communications temporarily today when cosmic radiation disabled a relay satellite. Although their mission has been marked by technical breakdowns, the astronauts reacted with human resourcefulness and characteristic coolness. They and their helpers on the ground quickly activated an alternative network for relaying human messages and much of the scientific data, allowing them to press home their program aimed at unraveling mysteries about the earth, its atmosphere and its oceans. So far in the mission the seven astronauts, the largest crew ever launched into space, have solved a series of minor but vexing problems, including a wobbly communications antenna and a balky radar panel.

On May 18, 1981 two days after Mike Wallace interviewed Gen. William C. Westmoreland for the CBS documentary ''The Uncounted Enemy: A Vietnam Deception'' - the producer of the program wrote Mr. Wallace a note of praise. ''The interview was a classic,'' said George Crile, the producer, who had composed most of the questions Mr. Wallace asked General Westmoreland, the commander of United States forces in Vietnam from 1964-68. ''Now for the reaction,'' he continued. ''I can't imagine Westie taking this lying down.'' Nor did the general.

A picture caption on the Music page in Sunday's Arts and Leisure section misstated the day of Myung-Whun Chung's conducting debut with the New York Philharmonic. It will take place tomorrow.

Dawn's first glow had yet to break over Salina Street here, but groups of men in plaid woolen shirts and down vests were already crowding into the Salmon River Sport Shop. Outside, in the darkness, pickup trucks and campers, their roofs a clutter of fishing rods, bedrolls and plastic buckets, jammed the streets of this village - that's pronounced pull-AS-sky, son - that straddles the Salmon River about 30 miles north of Syracuse. Still more vans and pickups, with license plates from a dozen states, packed the parking lot of the C & M Diner down the road a ways. New York's annual salmon run had begun.

NEW scientific evidence has raised the possibility that acquired immune deficiency syndrome, or AIDS, may be transmissible through saliva. The evidence, based on human and animal studies, is no more than suggestive in implicating saliva. But researchers said in interviews yesterday that they are convinced the studies raise real public health concerns. So far, however, none of the more than 6,000 cases of AIDS reported to public health officials have been directly attributed to infection from saliva. The research has been conducted both among humans and animals.

President Jose Napoleon Duarte of El Salvador offered today to hold peace talks with guerrilla leaders starting next Monday. Mr. Duarte, who made the proposal at the end of a 55-minute address to the General Assembly, said he was prepared to begin the negotiations in La Palma, a town near the Honduran border in an area of frequent clashes. In Mexico City, senior Salvadoran guerrilla leaders said they were ''completely surprised'' by the offer, but ''open and attentive'' to the prospect of such a meeting. U.S. Not Told of Topic One said they were awaiting ''private and formal'' communications. They noted that it was the first time Mr. Duarte had specified a time and place for talks with rebel representatives. (Page A10.)