What was going on when I was born?

After nearly a decade of selling off unprofitable business lines, closing plants and investing heavily in new lines of aerospace and military products, the Singer Company appears to have completed what some analysts call one of the most thorough corporate restructurings in recent memory. The changes have been slow and agonizing. What started in the mid- 1970's as a gradual retreat from its old-line sewing machine business has become, during the last five years, a huge overhaul of Singer's corporate identity. ''This is a company whose whole culture was built around the sewing machine,'' Joseph B. Flavin, the company's chairman and chief executive, said in a recent interview at Singer's headquarters here. ''As hard as we tried, that couldn't be changed overnight.''

The Government said today that air bags or automatic seat belts would have to be provided on all new passenger automobiles sold in the United States in five years. However, this requirement would be rescinded automatically if state legislatures representing two-thirds of the population required that occupants of autos use seat belts. The decision, after 15 years of controversy, was applauded by the auto industry but criticized by insurance companies and consumer activists. The reason for the opposition was the condition regarding state laws allowed by the Transportation Secretary, Elizabeth Hanford Dole. Law Passed in New York So far, only New York State is close to a law on seat belts. On July 2, the Legislature passed a bill that would require drivers and front-seat passengers to wear seat belts by the end of the year. Governor Cuomo said last week he was awaiting action by the Transportation Department before he dealt with the bill, but his staff said he was expected to sign it this week.

A report in New York Day by Day yesterday on groundbreaking ceremonies at Steeplechase Park in Coney Island misattributed a quotation. It was Comptroller Harrison J. Goldin who said, ''We're not doing the Parachute Jump yet, but that's next.''

Walter F. Mondale says he will not run on a ticket with the Rev. Jesse Jackson because of deep differences between them. Mr. Mondale made the remarks about Mr. Jackson, who complained Tuesday that he had not been considered for the Vice Presidency, in an interview to be broadcast Thursday on the ''CBS Morning News'' and made available here tonight. ''I intend to pick the best person I can find who's compatible, whose selection speaks for my vision of our future, and there are sufficient differences between Reverend Jackson and myself that I can't do that,'' Mr. Mondale said in the interview, excerpts of which were broadcast this evening. 3 Key Differences Cited In Minnesota, Mr. Mondale's press secretary, Maxine Isaacs, summarizing the former Vice President's interview, said the differences between the two men involved Mr. Jackson's call for a Palestinian state, relations with the Palestine Liberation Organization and cuts in the military budget. In Chicago, Mr. Jackson said that he accepted as ''honorable'' the reasons Mr. Mondale did not consider him as his running mate and that Mr. Mondale's selection process was ''good on balance'' and even ''commendable.'' (Page A16.)

Since martial law was imposed in Poland in December 1981, letters smuggled out of prison by Adam Michnik, an adviser of Solidarity, have helped sustain hope in the outlawed movement. Mr. Michnik is scheduled to go on trial Friday. It is a trial that the authorities tried to avoid by offering him freedom and the right to go abroad. It is a trial that Mr. Michnik demanded. The smuggled letters by Mr. Michnik, a 38-year-old activist, have been copied by hand, typed, mimeographed or printed in clandestine shops. They have been widely read and discussed.

THURSDAY, JULY 12, 1984 The Economy Sweeping new protection for the U.S. steel industry was recommended by the International Trade Commission. It voted, 3 to 2, to urge President Reagan to impose stiffer quotas and tariffs for five years on 70 percent of the steel entering this country. Such protection could lead to higher prices for steel and steel products in this country, experts say. (Page A1.)

The trial of a Solidarity adviser, Adam Michnik, whose defiant inspiring letters from prison have helped sustain hope in the outlawed labor movement since martial law was imposed in 1981, was scheduled to begin Friday. It is a trial that the Polish authorities reportedly tried to avoid by offering Mr. Michnik his freedom, but one that the 38-year-old activist demanded. (Page A1, Columns 2-3.) The Ambassador to the Vatican, William A. Wilson, was permitted to retain positions on the boards of two corporations when he was raised to that rank in January, according to State Department officials. The department could not provide an example of an ambassador who had been granted such an exception to the general policy requiring ambassadors to step down from directorships in profit-making companies. (A1:1-2.)

A report in the Discoveries column of The Living Section yesterday gave an incorrect address for Jonal. The store is at 17 East 67th Street.

The Dining Out Guide in Weekend on Friday gave an incorrect rating for the Homestead Inn. It was given three stars on March 18, 1984.

Walter F. Mondale's campaign chairman, James A. Johnson, said tonight that Mr. Mondale would announce his Vice-Presidential running mate on the Democratic ticket at noon Thursday at the State Capitol in St. Paul. Mr. Johnson told reporters, ''Earlier this evening, Vice President Mondale came to a decision about who his running mate would be in the 1984 election.'' There were several indications late tonight that the choice was Representative Geraldine A. Ferraro of Queens. The Associated Press, quoting unidentified Mondale aides, reported that she had been selected as his running mate. And it was learned tonight in San Francisco that Mrs. Ferraro had a reservation on a Northwest Airlines flight from San Francisco to Minneapolis at 12:40 A.M. Thursday morning.

Remarkably, in the face of the economy's continued rapid growth, the stubbornly high inflation that blighted the American economy for more than a decade has dissipated, and a vast majority of the nation's economists see little reason for it to return for as far as they can predict into the 1980's. Prices continue to rise, of course, but at lower rates than the country has seen since the 1960's. Some experts say the United States may have entered a long-lived period of disinflation in which a broad range of prices would increase at a rate significantly slower than in the past. Economists attribute this slowing of inflation partly to the determination of the Federal Reserve to discourage rising prices - by restraining growth of the money supply, starving demand for credit and letting interest rates soar - even at the cost of the long 1981-82 recession. Convergence of Forces Economists also cite the fortuitous anti-inflationary convergence of several other forces: the surprising strength of the dollar, softening prices for vital raw materials such as oil, food and other commodities, a low level of wage increases, and deregulation of such industries as airlines and telecommunications.

Ten New York City water and sewer inspectors were charged yesterday with taking bribes for certifying work on private homes and other buildings without properly inspecting them. The inspectors were accused by Federal and city officials of taking bribes of between $5 and $20 on 52 occasions from a private contractor who was cooperating with investigators. The 10, who are among the 25 inspectors in the water and sewer divisions of the Department of Environmental Protection, were the only ones investigated and were picked at random, according to city officials. The officials said none of the 10 had refused a bribe during the yearlong inquiry.