What was going on when I was born?

A Pentagon inquiry into the bombing of the Marine compound in Beirut on Oct. 23 has concluded that serious failures in the Marine chain of command contributed to the tragedy, according to Reagan Administration officials. One official said that ''generals and admirals'' in the European Command were included in the ''sharp criticism.'' (Page A1, Col. 6.) The U.S. Marines and French troops in Beirut were warned in an anonymous telephone call to leave Lebanon within 10 days or face a major attack. The caller said he represented a Moslem terrorist group. (A8:1-2.)

Plans for a 93-acre Westway State Park to run along the Hudson River from Battery Park City to 34th Street were announced yesterday by state officials. The park, with plazas, esplanades, playing fields, lawn areas and benches overlooking the waterfront, would be built over the proposed underground highway. State Transportation Commissioner James L. Larocca and State Parks and Recreation Commissioner Orin Lehman announced three alternative designs for the park. They will be reviewed in the next few months by community and park groups, and a final design will be drawn up.

''The Christmas message in Harlem, the good news to the poor, is that 'you're not going to be poor anymore.'

The Securities and Exchange Commission today approved on a 4-to-0 vote an accounting device to let companies dress up their financial condition and show higher earnings by wiping long- term debt off their balance sheets. The device, called in-substance corporate defeasance, allows companies to buy Government securities to reduce their balance sheet debt by an equal amount. Analysts said the technique would appeal primarily to companies with cash to spare and a lot of low-interest debt on their books.

For weeks, if not months, Spaniards have been waiting breathlessly for the arrival of ''El Gordo'' - ''The Fat One.'' And they are not talking about Santa Claus. Today, at 9:21 A.M., El Gordo arrived, a bit earlier than usual. There was the customary pandemonium: flashes from the photographers' lights, cheers and cries, a mad dash for the telephones. Throughout the country people who had gathered around television screens dipped into their wallets and purses and held up bits of paper, re-examining numbers already memorized.

A film review yesterday of ''Malou'' incorrectly listed the theater where it is playing. It is Guild's Embassy 72d Street 1, on Broadway.

A young engineer was recently recruited by a major computer maker here. His house back home in South Carolina was worth $67,000, but the engineer and his wife could find nothing comparable near his new job here for much less than $235,000 - far more than they could afford on his new salary of $45,000. As a result, his new employer is faced with the prospect of subsidizing the engineer's housing to the tune of $25,000 over the next five years or losing him. The case illustrates the growing housing crunch in this small strip of land south of San Francisco that in recent years has become a caldron of industrial innovation and has become known as the Silicon Valley, after the material used in the semiconductor chips that are the backbone of its high- technology industries. Many of the area's corporate and civic leaders are beginning to worry that mounting congestion and housing costs are contributing to the deterioration of the valley as a place to create new ideas and to do business.

Yasir Arafat, who was evacuated from northern Lebanon Tuesday with 4,000 of his followers, met in Cairo today with President Hosni Mubarak. It was Mr. Arafat's first visit to Cairo in six years. The Palestine Liberation Organization, which he heads, and 17 other members of the Arab League broke relations with Egypt because Egypt signed peace agreements with Israel. The two men embraced on the steps of the presidential palace, then met in private for nearly two hours.

The Reagan Administration praised the meeting between Yasir Arafat and President Hosni Mubarak of Egypt today as ''an encouraging development'' toward reviving stalled peace efforts in the Middle East. Approving statements were issued by the White House and the State Department only hours after Prime Minister Yitzhak Shamir of Israel denounced the meeting between Mr. Mubarak and the Palestine Liberation Organization leader. The Administration's endorsement of the meeting produced further expressions of unhappiness by the Israelis, who said the Americans were acting contrary to the interests of peace by seeming to enhance Mr. Arafat's prestige, only days after he was forced to leave Lebanon. The open dispute between Israel and the United States on such an important issue underscored the fragility of the new ''strategic cooperation'' between the two countries, which was highly publicized a month ago when Mr. Shamir was in Washington.

There is a man in East Harlem called ''black Jesus'' by many in the neighborhood. The Rev. William James said such a description of him was foolishness, but admirers tell tales of his works as remarkable to them as some in the Bible. On one of his frequent walks among the panhandlers, thieves, drug addicts and prostitutes of the city, Mr. James paid the price of a Times Square prostitute, asking in return only that she call him. Intrigued, she did call, and when they met, he learned that the young woman had once wished to be a teacher.

Companies The F.T.C. ruled that General Motors and Toyota could build Japanese-designed cars in California without violating antitrust laws. The divided 3-to-2 vote came over opposition from G.M.'s domestic competitors, and in Detroit, Ford, Chrysler and American Motors immediately denounced the decision. But the ruling could lead to similar pairings between Chrysler and other foreign manufacturers. (Page A1.) A Federal court jury found three companies , Phelps Dodge Industries, Cerro Copper Products and Cambridge-Lee Industries, not guilty of conspiring to fix copper tubing prices. The jury also acquitted six executives. (D1.)

The Federal Reserve Board proposed yesterday that American banks be required to write off portions of loans to countries that are deemed by banking regulators to be in serious financial trouble. Similar proposals are being issued by the other Federal bank regulators. The proposals reflected a new move by regulators, mandated by Congress, to impose judgments about the quality of foreign loans. Banking sources said the regulators had indicated that, under the proposals, banks would be required to write off 75 percent of the value of their loans to Zaire, 50 percent of their loans to the Sudan, 15 percent of those to Poland and Nigeria and 10 percent of those to Bolivia. The Fed did not confirm that any of these debtor nations were involved.