What was going on when I was born?

Nestled in a lushly upholstered chair in his Park Avenue office, Toshihiro Tomabechi, the 65-year-old president of the Mitsubishi International Corporation, marveled the other day at how much things have changed since he first came to the United States in 1951. Then, Mr. Tomabechi was one of a few dozen Japanese businessmen and diplomats in New York. He represented one of the Mitsubishi group's ''splinter firms,'' created in 1947 when the Allied command broke up the powerful Japanese families of companies known as zaibatsu, which literally means ''wealthy clique.'' In 1951, with World War II still a vivid memory, anti-Japanese sentiment was widespread and, by law, Japanese stationed in America were prohibited from bringing their families here. ''It was not until May 1953 that my family could join me,'' Mr. Tomabechi recalled.

The Canadian Air Traffic Controllers Association announced last night that its members would no longer handle air traffic to and from the United States, except in emergencies. The union said it had acted because of what it regards as unsafe conditions created by the six-day-old strike of American controllers. At a meeting in Ottawa, the union announced that beginning at 7 A.M. today scores of flights to or from the United States "would not be processed." The union cited what it said were 41 documented incidents of aircraft flying too close together and nine instances of "evasive action" taken by aircraft in Canadian airspace. It also referred to "amateurish attempts" in the United States to run an air traffic control service" with "unqualified substitute personnel."

Water in America: Solving the Quandary Second of five articles While New Yorkers suffered through a drought emergency this spring and reservoir levels sank dangerously low, basements in parts of Queens were flooded with some of the 1,500 billion gallons of water that lies, mostly untapped, beneath that part of the city. At the same time, as millions of New Jersey and Connecticut residents underwent mandatory water restrictions, billions of gallons of water were overflowing from reservoirs in the area and running off into the ocean. In July vandalism cut off Newark from its major reservoirs and left the city of 600,000 people with less than two weeks of water. Engineers worked day and night to restore the rupture and complete pipes to nearby water systems that, almost all agree, should have already been in place. According to most experts, the towns and cities in New York metropolitan area have, at present, no water supply problems so great that they cannot be solved with careful planning, basic conservation measures and better systems of maintenance, control and coordination.

Three months after the start of a 50-cent surcharge on nighttime and Sunday taxi fares, the Taxi and Limousine Commission has been unable to get any data from the independent cab drivers on whether more drivers have been encouraged to work during those times. Without this data from the independent drivers, who lobbied hard for the surcharge, the 50-cent nighttime and Sunday surcharge may be scuttled, the chairman of the commission warned yesterday. The official, Jay L. Turoff, said the fleet owners - representing only 20 percent of the city's 11,787 medallions -had provided statistics showing that drivers had been making an average of 2.3 extra trips each evening and that the number of cabs remaining in garages for lack of drivers had been cut in half. But he said he could not draw definite conclusions from those numbers. And he said the commission did not have the manpower to do a complete survey of its own on the need for the extra taxis.

International One of the toughest Soviet attacks on the Reagan Administration followed the report over the weekend that the United States would proceed with the full production of neutron weapons. Tass, the official Soviet news agency, said the American move was ''a step designed to bring the world closer to nuclear catastrophe,'' and that the Soviet Union would respond in the manner required by its own security interests. (Page A1, Column 6.) West Germany responded cautiously to the Reagan Administration's plans to produce neutron weapons. A Government spokesman said that the decision was an ''exclusively American affair.'' Influential members of Chancellor Helmut Schmidt's Social Democratic Party who believe that the Administration is not taking disarmament efforts seriously indicated there would be increased resistance to support of its armaments policy. (A6:3-6.)

Developing countries have made considerable progress helping themselves, yet they face an uncertain future because of the slower growth of world trade and less generous aid policies of industrial countries, the World Bank said today. In its fourth annual world development report, the bank drew attention to the ''increasingly desperate predicament'' of the poorest countries and made a plea for open trade policies as the catalyst for growth for both the industrialized world and the developing world. The 192-page analysis was prepared by the staff of the International Bank for Reconstruction and Development (the World Bank's official name) under the direction of Robert S. McNamara, who retired as its president June 30. Data from the report will be studied by industrial and developing countries before their conference at Cancun, Mexico, in October.