What was going on when I was born?

John W. Hinckley Jr. pleaded not guilty today to charges of attempting to kill President Reagan and assaulting three other men last March. Mr. Hinckley announced his plea in a firm, clear voice as he stood before Judge Barrington D. Parker in a heavily guarded courtroom for arraignment. With the concurrence of prosecutors and defense attorneys, Judge Parker signed an order declaring that the 26-year-old defendant was mentally competent to participate in the arraignment and related proceedings in Federal District Court here. Mr. Hinckley ''has a rational and factual understanding of the charges against him and is able to assist his attorney in the preparation of his defense,'' the order said. Deadline for Insanity Plea The judge gave the defense attorneys until Sept. 28 to file a formal notice if they intend to use an insanity defense or to introduce expert testimony to the effect that Mr. Hinckley had some mental disease or defect. No trial date was set.

Mexico and France recognized El Salvador's guerrilla-led opposition today as a ''representative political force.'' The joint declaration, handed this morning to the current president of the United Nations Security Council, Jorge Illueca of Panama, runs counter to the Reagan Administration's policy. Washington seeks the military defeat of the guerrillas and urges democratic opposition parties to take part in elections for a constitutional assembly scheduled for next March. The United States was not mentioned by name in the statement.

Secretary of State Alexander M. Haig Jr. accused the rebels in El Salvador today of engaging in ''straight terrorism'' and again accused Cuba and the Soviet Union of playing major roles in fomenting the country's civil war. ''We've witnessed the guerrilla movement resort to straight terrorism, the kind of activity which reflects their failure and frustration in major force operations,'' he said. ''They have now gone into an effort in which the main victims of their activity are innocent noncombatants, the people of El Salvador.'' He referred to a campaign in thecountry to destroy power lines and disrupt water supplies.

Lieut. Arthur Dallas's longest day in the Police Department began when he fired two shots from his service revolver at a man who was waving a pistol at motorists on a South Bronx Street in June. The man, James Robertson, was killed, and the pistol turned out to be a plastic toy. ''I felt lonesome,'' Officer Dallas said later. ''You recognize you are out there on your own and you made a decision in a life-and-death matter. For policemen, this is the difficulty of the job precisely.'' Though there have been a number of such controversial shootings by police officers this year, both officials in the department and its critics say the New York City police show more restraint than the police in other major cities.

The Justice Department reversed itself today and said that it was satisfied with this city's proposals to desegregate its schools. On July 21, the department assailed the school board proposals for desegregating the nation's second largest school system as ''incomplete.'' The Cabinet agency said the city's plan placed an unfair burden on black students in that it would leave many schools as much as 70 percent white when white students make up only 18 percent of total enrollment, and would leave about 250 schools all-black or all-Hispanic American. Misunderstandings Suggested But in a joint statement submitted to the United States District Court, which is overseeing desegregation efforts here, the Justice Department did an about-face. It suggested that its criticisms had been ill-advised and the result of misunderstandings between the Government and Chicago school officials.

Christianne Schoedel and Barnaby Starr were married yesterday in the Finch Arboretum in Spokane, Wash. The Episcopal ceremony was performed by the Rev. Robert R. Coombs, dean of the Cathedral of St. John the Evangelist in Spokane.

A 153-pound female whale born five days ago at the New York Aquarium is already something of a star because she is only the fourth beluga whale born in captivity. But she is causing some worries because none of the others survived and she is refusing her mother's milk. The calf's mother, Kathy, who weighs about 1,500 pounds, gave birth at 7:25 A.M. Monday. The midwife, Bob Green of the aquarium, plunged into the cold waters favored by these arctic creatures to bring the young light-gray calf to the surface immediately to start her breathing properly. One of the other beluga whales born here had failed to do so, according to aquarium officials.

The Government index intended to forecast movements in overall economic activity fell by one-tenth of 1 percent in July, the Commerce Department reported today. Although this was the third consecutive decline in the index of leading indicators, a development often taken as presaging a recession, the slightness of the decline was regarded by economists as mildly encouraging. ''That's probably a good result,'' said Robert Ortner, senior vice president of the Bank of New York. ''The index is telling us the economy is stalled, but I don't think it's pointing to collapse.''

Administration officials are increasingly concerned that European members of NATO, in the face of growing opposition at home, may not be able to carry out the alliance's decision to start deployment of modernized American medium-range nuclear weapons in late 1983. A series of conversations with foreign policy, military and intelligence officials showed that there has been deterioration in the last six months of the chances of success for the so-called theater nuclear force program for Western Europe. The Administration regards the program as central to maintaining the vitality and cohesiveness of NATO over the next decade. Europeans have said that the confrontation policies of the Reagan Administration, and its decision to produce neutron weapons, are major reasons for the Western European opposition to the program. But officials here believe that the problem is more one of weakened governments within the alliance and an insufficiently forceful approach by their leaders in presenting public opinion with the negative realities of the changed balance of power between NATO and the Soviet Union.

The American SR-71 reconnaissance plane that was fired at on Wednesday from a North Korean ground-launched missile site first detected the attack through a radar warning device, an Administration official said today. The Lockheed-built aircraft has electronic equipment that allows it ''to determine when it comes under radar surveillance,'' the official said. Yesterday a Pentagon spokesman, Henry Catto, said that the pilot of the SR-71 had seen the contrail of a surface-to-air missile launched from North Korea and the smoke of its explosion.

Prime Minister Pierre Elliott Trudeau defended the security service of the Royal Canadian Mounted Police today against some of the charges of illegal activities brought by a Government commission. He suggested at a news conference that violations of traffic rules, secret entry into private homes, use of false identities and minor thefts could constitute an essential part of police work. Solicitor General Robert Kaplan, the minister in charge of the Mounted Police, made a similar point yesterday at a meeting of chiefs of police in Edmonton, Alberta. He said it was ''a total misnomer to consider these practices illegal.''

North Korea today denied American charges that its forces fired a missile Wednesday at a United States SR-71 reconnaissance plane flying in what the Pentagon called ''South Korean and international airspace.'' But North Korea said nothing about whether such an attack had been made in what it defines as its own airspace. Issuing its first direct statement on the incident, the official North Korean press agency charged that the SR-71 had violated ''the territorial air of the northern half of our republic,'' meaning North Korea. The statement did not elaborate.