What was going on when I was born?

The heavy rains and balmy, snow-melting temperatures of midweek combined to push water reserves in New York City's three reservoir systems to nearly 40 percent of capacity, officials said yesterday. Francis X. McArdle, the city's Environmental Protection Commissioner and head of its water-conservation program, said: ''No one should interpret this good news as a sign that the drought is over. It gives us only another two weeks of life. We have a long way to go.''

President Reagan has rejected a plan to make his proposed 10 percent cut in personal income taxes retroactive to Jan. 1 and instead plans to ask Congress to begin the decrease on July 1, Administration sources said today. Mr. Reagan still plans, however, to propose that increases in depreciation allowances for business be retroactive to Jan. 1 so that businesses will not delay investments that the Administration hopes will stimulate the economy. The decision came to light today as Mr. Reagan heard the final budget-cutting proposals of his Cabinet officers and budget director in a series of meetings at the White House. Mr. Reagan also overruled as politically undesirable a last-minute attempt to decrease the tax on investment income. Accepts Foreign Aid Compromise In addition, the President ordered further economies in the military budget, which he intends to increase, and accepted a compromise foreign aid proposal that seemed to mark an end to the dispute between Secretary of State Alexander M. Haig Jr. and David A. Stockman, the budget director.

The New York City Buildings Department no longer makes enough inspections of construction projects to know whether fire and safety codes are being met, according to department officials, architects and engineers. They also say that much construction is done without permits and that such jobs, in addition to creating potential hazards, cost the city millions of dollars a year in lost fees. The Buildings Department, because of legal and policy changes and a reduction in its employees caused by budget cuts, has turned over much of its traditional role of protector of the public to the various professions in the construction industry. Some Cutting of Corners Some officials, architects and engineers have asserted in interviews that under this honor system builders and contractors have sometimes cut corners by using inferior materials and doing some work haphazardly or not at all. Concern was also expressed in the interviews that some in private industry who are charged with doing inspections are routinely approving work they have not inspected or know to be in violation.

The prices that producers charge retailers for finished goods rose by nine-tenths of 1 percent last month, a significant acceleration from the pattern of last fall, the Labor Department reported today. The principal cause of the acceleration was rising prices for fuel oil and gasoline, which are expected to rise even more sharply in coming months because the figures released today did not include the effects of President Reagan's decontrol of domestic oil on Jan. 28. In December, the prices charged by producers of finished goods rose by five- The dollar surged in value in both Europe and the United States amid hectic buying. Page 29. tenths of 1 percent. The increase had been seven-tenths of 1 percent in each of the two preceding months.

The four charities that were given $1 million each by the Department of Energy just before the Carter Administration left office said today that they had already distributed much of the money. They expressed dismay at attempts by the Government to get it back. The money, from the settlement of suits alleging overcharges by the Standard Oil Company (Indiana), more familiarly known by the name of its gasoline, Amoco, was provided to the charities to help the poor pay fuel bills. The distribution was ordered by Paul L. Bloom, who was the Energy Department's special counsel. He defended his action today, saying that the money could in no way be considered taxpayers' money.

The Reagan Administration has decided to make what it sees as Soviet-bloc support for El Salvador's insurgents an early test of relations with both its allies and the Soviet Union. The decision and the background for it have yet to be officially announced, but in coming days and weeks the Administration will be making known, first to foreign governments, then to Congress and the public, the intelligence data that it says will show conclusively that the Soviet Union, Vietnam, Cuba and other bloc countries have secretly aided the left-wing guerrillas trying to overthrow the Government in El Salvador. It is not possible to evaluate independently the Administration's case in the absence of the secret intelligence information. But what is clear is that the Reagan Administration - pushed on this issue by Secretary of State Alexander M. Haig Jr. - has chosen to bring El Salvador to the forefront of world attention and to make attitudes toward the guerrilla war there an early barometer of relations with the United States.

Longtime residents say there are at least three things worse than December in Peking: January, February and March. Dusty, sooty, cold and dry, Peking in winter is like living in a vacuum-cleaner bag in a freezer. It is the kind of place that makes Americans homesick for overcast Februarys in Des Moines. New Yorkers, enduring their own cold and dry winter, will find no sympathy here, though they may be comforted to know that the inhabitants also see every breath of air they take. A year ago, on a seemingly clear day, Stephen Gage, a member of a visiting delegation from the United States Environmental Protection Agency, set up portable air-quality measuring equipment near the Peking Hotel. He recorded 150 microns of particulates per cubic meter of air. In the United States 25 microns - a micron is 0.000039 of an inch - is considered safe. At least one foreigner wears an industrial air filter while jogging, saying that otherwise a two-mile run would equal a two-pack-a-day cigarette habit.

At least 40 people were killed and 200 injured in a fire that swept through a nightclub here early today, the police reported. Police and ambulance-service officials said that many victims of the blaze in the Stardust nightclub, in the Artane district of north Dublin, were apparently trampled during a rush for the doors when the fire started near the stage.

The threat of civil war appeared to be receding today after Prime Minister Robert Mugabe used what remained of the old Rhodesian Army and Air Force to crush rebellious former guerrillas with nominal loyalty to Joshua Nkomo, a member of his Cabinet. By tonight, 30 hours had passed since the last report of serious clashes. In all, it seemed likely that at least 150 people had died in the fighting.

Poland's new Prime Minister, Gen. Wojciech Jaruzelski, told the nation tonight in his first news conference that the Government wanted to keep open channels of communication with the people. The Prime Minister also called on news organizations to provide full information on what the Government was doing and planned to do. ''We hope you will never complain about a lack of information, but rather about an overabundance,'' General Jaruzelski told Polish journalists in the nationally televised news conference. General Jaruzelski, who has called for a 90-day moratorium on strikes, said that the news conference was a ''harbinger of things to come.''

An Indian Airlines plane with 121 people on board caught fire at a Madras airport today, the Press Trust of India news agency reported. No casualties were reported.

Corsican separatists claimed responsibility today for three Paris-area bombings during the night, including one that seriously damaged the Italian tourist office and another that destroyed 15 parked cars.