What was going on when I was born?

The Reagan Administration says the Salvadoran military has broken a long stalemate with guerrilla forces and is capable of neutralizing the insurgents by the end of 1986 if current military and political trends continue. ''There has been a recent convergence of developments that, if continued, will promote eventual success in getting the insurgents under control,'' the Under Secretary of Defense for Policy, Fred C. Ikle, said in an interview on Friday. Another senior Pentagon official at the interview said, ''The Salvadoran Army has turned the corner.'' Independent military experts questioned the optimistic assessment, saying that the war remains a stalemate and that the Salvadoran Army, while improved, cannot defeat the guerrillas.

At long last, Fordham Plaza, the mixed-use office and retail complex, planned more than a decade ago for a 5.5-acre urban renewal site on Fordham Road in the Bronx, appears to be a reality. On Tuesday Mayor Koch will put shovel to earth at Third Avenue and Fordham Road, symbolizing the start of construction at the $60 million private and public development project.

A SWEEPING bill to discourage movies, magazines and books that depict sexual violence against women has set off a constitutional furor in Suffolk County. The measure, which was proposed by Republican County Legislator Michael D'Andre of Smithtown, would allow any member of the Suffolk public to sue the makers and purveyors of what it defines as ''objectionable pornography'' that presents ''sexually explicit subordination of women.'' While it would not apply to nonviolent pornography, the bill brings within its scope a wide range of general distribution movies and books, including the Bible, according to one critic. Others, including County Attorney Martin B. Ashare, said the bill is patently unconstitutional and an ill-considered threat to the full array of free speech rights. There are also hints that County Executive Peter F. Cohalan will veto the measure if it is approved by the County Legislature. A vote could come as early as this month.

The seven astronauts aboard the space shuttle Challenger solved one annoying problem today and worked on another as they struggled to carry out one of the most ambitious programs of Earth observation ever. The astronauts were preparing to fire the spaceship's jet thrusters to bring it into a lower orbit for a fast- paced program of experiments when they encountered two vexing problems. First, a communications antenna on the shuttle began to wobble. A plan was quickly developed to switch off the antenna's steering motor and instead turn the entire spacecraft to point it at a satellite so that precious scientific data could be relayed to earth. Then, a radar panel would not stow properly, frustrating plans for the shuttle to change its orbit. The unfolded panel, 35 feet long and 7 feet wide, was one inch short of its latch.

THE young woman, a student at Douglass College who had always lived in the New Brunswick area, stared at the fountains where water cascaded over brick semicircular steps at Monument Square here. ''I remember what this city used to look like,'' she told a passer-by who had paused to share the view, ''and I think I'm in Paris.'' Above the fountains, a bronze Civil War soldier - the monument for which the square was named - seems to gaze beyond shrubbery and beds of geraniums to shoppers strolling along George Street, repaved in red brick and enlivened with stylish boutiques. Eventually, George Street will be closed to vehicular traffic, except for buses to shuttle students between the sprawling campus of Rutgers, the State University, to the west, and Douglass, to the east. Across the street from the square, a white Gothic church stands beside a modern brick-and-glass office building, further evidence of a new city rising among the symbols of the past. And where Livingston Avenue meets George Street, four buildings have been converted into a cultural center.

Alan S. Moore, the general manager of the Galaxy Towers condominiums overlooking the Hudson River in Guttenberg, N.J., keeps bus tickets in his safe. ''To me, they're just like money,'' he said. Mr. Moore's appreciation of the value of bus tickets is based on more than a fondness for that mode of transportation. Every day, nearly 300 residents of the Galaxy Towers - about one of every seven people who live there - board private buses bound for New York City with tickets purchased from Mr. Moore. At $125 for a book of 50 tickets, the buses involve a considerable amount of money and, for Mr. Moore, added responsibility. ''We even have an accountant who keeps track of sales,'' he said.

IT'S 7 A.M. and Joseph Duerr, in gym shorts, T-shirt and baseball cap, hops into his car for the 20- minute drive to his Melville office complex. Once there, Mr. Duerr, a vice president with Paine Webber Jackson and Curtis Inc., doesn't head straight for his desk. Instead, he shoots downstairs to the complex's gym and sweats out an hour of bench presses, weight lifts and cycling before showering and suiting up for the often stressful day ahead. Said a trim and tan Mr. Duerr, ''Virtually, I'm never without my gym bag - in the trunk or under my desk.'' At lunchtimes, farther out on the Island at Hauppauge, while other workers crowd the roads to get to the nearest restaurants, Tony Sturniolo, a 22-year-old cable assembler, prefers to stay put at Satellite Transmission Systems Inc. There, in a small carpeted room set off from the plant, Mr. Sturniolo, bare- chested and muscled, moves methodically around a nine-station Universal gym as a portable radio plays in the background.

The Producer Price Index, which has served as a security blanket for many inflation worriers since January, should show signs of fraying next Friday. Some economists, such as Jason Benderly of Kidder, Peabody & Company, say the September index could rise as much as one- half of 1 percent, compared with an increase of three-tenths of 1 percent for August, and no change at all for the previous three months. ''You can blame the rise mostly on the surge in oil prices that started in August, the month that the September figures actually measure,'' he said. ''By comparison, the Consumer Price Index for September, which is due out on the 24th, will be reflecting current prices.'' Mr. Benderly also noted that the P.P.I. only covers prices at the wholesale level, including imported goods, which are relatively cheaper because of the dollar's continuing strength. That ameliorating effect keeps the lid on the P.P.I. But the C.P.I. weighs the cost of goods and services at the retail level. Those services - such volatile items as medical costs, housing costs and repair bills - are increasing. Thus it is likely that inflation, as measured by the C.P.I., will be rising at an even faster rate than the P.P.I. indicates.

In one of the most gripping games in playoff history, the San Diego Padres defeated the Chicago Cubs, 7-5, tonight on a ninth- inning home run by Steve Garvey and tied the National League playoff at two games apiece. It was the second straight victory for the Padres, and they got it on the fourth straight hit by the 35-year-old Garvey. He put them in front with a double in the third inning, tied the game with a single in the fifth, put the Padres ahead again with a single in the seventh and finally won it with a home run off Lee Smith in the ninth. It was a game that kept the 58,354 fans roaring, as 10 pitchers struggled to hold leads that switched time after time. In the final frenzied innings, the Padres seized a 5-3 lead in the seventh, lost it in the eighth when the Cubs mauled Rich Gossage for two runs, survived a bases-loaded threat by the Cubs in the top of the ninth and finally prevailed when Garvey swung in the bottom of the ninth.

Whenever the Royal Shakespeare Company pays a visit to Broadway, it is, by definition, a major theatrical event. Usually the R.S.C. has been represented in New York by a single production, last season with ''All's Well That Ends Well,'' ''Good'' in 1982 and the year before by the landmark two-part version of ''The Life and Adventures of Nicholas Nickleby.'' This season the R.S.C. returns with two plays, ''Much Ado About Nothing'' and ''Cyrano de Bergerac'' (now in previews, opening Oct. 14 at the Gershwin), a repertory season that is more representative of the company's founding principle. The R.S.C. is arguably the finest ensemble classical repertory theater in the world today. Ensemble acting in repertory, especially in classics, is the essence of the R.S.C., as exemplified in the current instances by Derek Jacobi shifting from the misogynistic Benedick in ''Much Ado'' to the romance-obsessed Cyrano and by Sinead Cusack doubling as the acid-tongued Beatrice and the adoring Roxanne. However, even as theatergoers admire the versatility of the actors, they should be aware that what is being seen on Broadway still represents only a slim sampling of the work of a company that is unmatched for its productivity and for its reserve strength.

A yellow cloud of noxious fumes leaked from a chemical insecticide plant on the Arthur Kill yesterday and spread over a 20-mile area of New Jersey and Staten Island, causing alarm and physical irritation among thousands of residents but no serious injuries. The fumes, a vaporous byproduct of a widely used insecticide called malathion, escaped from a storage tank at the American Cyanamid Company plant in Linden, N.J., shortly after 10 A.M. The fumes made scores of people ill, including 29 seaman aboard a passing freighter, 11 others on a tanker in Arthur Kill and about 15 men working on nearby docks in Carteret, N.J. More than 100 people were treated at hospitals for effects of the fumes. Workers Evacuated As the cloud drifted south, the fumes also prompted the evacuation of hundreds of workers from shoreline industrial areas and produced thousands of calls to police departments and hospitals from worried residents in Union and Middlesex Counties in New Jersey and on the southern end of Staten Island. The police on Staten Island and in Carteret, Woodbridge, Perth Amboy, South Amboy, Linden, Edison, Sayreville and Old Bridge in New Jersey were deluged with calls from people complaining of burning eyes and skin and sickening odors. The police explained the source of the fumes to callers and advised that infants and those with respiratory problems should stay indoors or leave the affected area temporarily and consult doctors.

LAKE FOREST, Ill. WALTER PAYTON was sitting in a meeting room at the Bears' training complex, here in this affluent community north of Chicago. It was rather unusual that he would remain stationary for so long; it's not his nature. The day before, while his wife, Connie, and their 3-year-old son, Jarrett, went to see the Cubs beat the Padres in the first game of the National League playoffs, Walter Payton went hunting for deer with bow and arrow. ''He likes baseball,'' says Tracy Nguyen, his personal secretary. ''But he's hyperactive. He can't sit still long enough to watch it.'' On this day last week, he was in place long enough to wrestle with a concept that, clearly, makes him uncomfortable. This business of ''the greatest running back who ever lived,'' Payton or Jim Brown. For the last several months it has been a hot topic in the Chicago area, or, as hot a topic as there could be, given the wondrous summer played out by the Cubs. With 67 yards of rushing today, when the Bears engage the New Orleans Saints at Soldier Field in Chicago, Payton will break Brown's National Football League record of 12,312 career rushing yards. With 100 yards, he would break another Brown record, most games with 100 yards or more rushing. At the moment, Payton and Brown are tied with 58. Great. Greater. Greatest. Payton looked perplexed.