What was going on when I was born?

The Department of Justice announced yesterday that it would file a civil antitrust suit to block a proposed merger of the second- and third-largest pay-television networks, Showtime and the Movie Channel. The department said the combination of the two pay-television channels would stifle competition in the sale of their programs to cable systems. It also said that having three competing film studios in business together could hamper competition in their sale of films to pay-television channels. The proposed merger would be a joint venture of three film studios - Paramount Pictures; Warner Brothers, a division of Warner Communications Inc., and MCA Inc. - and two companies involved in supplying entertainment by the use of satellites, the American Express Company and Viacom International.

The Labor Party emerged from Thursday's general election in no condition to offer itself as an alternative government. It must now struggle to remain the principal opposition force, struggle for its very survival. That was the widespread conclusion among professional politicians, academics and journalists today as they surveyed the outcome of the voting. Unable, according to computer calculations, to win even half of the vote of the trade unionists whose interests it was formed to defend, unable to poll as much as an eighth of the vote in more than 100 of the 650 constituencies, the party found itself forced back into its redoubts in the inner cities and the mining valleys.

Prime Minister Margaret Thatcher promised today that her landslide victory in Thursday's general election, which gave her the largest majority of any British Government since 1945, would not prompt her to impose extreme right-wing policies. In a series of television interviews the Prime Minister said her ''new look'' Cabinet, which is to be announced Saturday, would ''reflect the whole spectrum of views in the party.'' The opposition parties suggested during the campaign that a Conservative sweep would lead to the disappearance of Tory moderates, known as ''wets,'' from the Government. ''I have not been extreme for the last four years,'' Mrs. Thatcher, 57 years old, said. ''I am not an extreme person, and I won't be extreme now. All power is a trust; we have to use our power wisely and well.''

A new church for the nation's more than three million Presbyterians was created here today, ending a North-South split that dated from the Civil War. The Presbyterian Church (U.S.A.) came into being as 12,000 people gathered for a holy communion service that was transmitted by satellite to 24 places around the country. Earlier today, 1,000 delegates to the General Assemblies of the United Presbyterian Church in the U.S.A., the Northern wing, with 2.4 million members, and the Presbyterian Church in the United States, the Southern group, with 815,000, voted their denominations out of existence and celebrated the creation of the new church with a parade down Peachtree Street. 3 Million in 12,000 Churches The merger, which Presbyterians here speak of carefully as a ''reunion,'' heals a split that occurred in 1861, when Presbyterians in the Confederate States severed ties with churches in the North. The new denomination will have almost 3.2 million members in 12,000 churches, making it the fourth-largest Protestant denomination in the United States, after the Southern Baptists, the United Methodists and the National Baptist Convention, a largely black denomination.

Two meetings of the Soviet leadership here next week are expected to throw light on key questions that have arisen about the health of Yuri V. Andropov and his hold on power in the seven months since he became leader of the Communist Party. Recently, he has not acted so forcefully nor looked so vigorous as he did in his first weeks in power. There are growing doubts among Western diplomats and analysts about whether he has the physical strength or political clout to move any more people of his choice into positions of power than he already has, and whether he can break the bureaucratic inertia responsible for the sluggish state of the country's economy. Unless other events intervene, the party's Central Committee will meet for its regular summer session, a session that might authorize new appointments to the bodies exercising control - the Politburo and the Secretariat. The Central Committee session will be followed on Thursday by a meeting of the Supreme Soviet, the country's nominal Parliament, which has the power to fill the post of chief of state, vacant since Leonid I. Brezhnev died in November.

A convict who has spent 25 of his 41 years in prison was sentenced today to die in the electric chair for the murder of a corrections officer. The execution of the convict, Lemuel W. Smith, was set for the week of July 10. But it will be stayed automatically as soon as Mr. Smith's attorneys file an appeal with the State Court of Appeals, New York's highest court. The appeal must be filed within 30 days.

B.B. King, the blues singer and guitarist, came home to Indianola this weekend, and blacks and whites celebrated the occasion together. The celebration was remarkable. Unlike other places North and South, there is not much social mingling between the races in this town in the heart of the Mississippi Delta. In a ceremony today, the town renamed the two-lane street that runs past Gentry High School B.B. King Road. A variety of festivities have been taking place. They will culminate in a concert Saturday night near the plantation where the 57-year-old bluesman once toiled as a common laborer.

The Federal authorities investigating the gun battles that led to the death of the tax protester Gordon W. Kahl say they are uncovering troubling links among small groups of heavily armed right-wing extremists. The ties cut across organizational lines from the Nazi Party and the Ku Klux Klan to radical elements of the farm protest movement. Adherents are armed in some cases with explosives, grenade launchers and automatic and semi-automatic weapons and trained in guerrilla warfare. They are said by these authorities to be bound by anti-Semitism, anti-Communism, religious tenets and beliefs that taxation is illegal and that currency is debased by an amorphous conspiracy of Jews, bankers, Communists, their dupes and corrupt government officials. The link among the disparate right-wing groups is not organizational but ideological and theological. Federal officials said they could not say that a formally organized national network existed.

The comings and goings of ambassadors are not usually of much moment in Bonn. But a few days ago Sven Backlund left the post he occupied at the Swedish Embassy for more than a decade, depriving West Germany's capital of one of its wittiest aphorists and one of its true insiders. ''You can amuse yourself with a specialty; you can amuse yourself with stamps, with silver,'' the lanky, big-boned Swede said. ''I amused myself in knowing things.

A great wall of sand stretches across the deserts of Western Sahara these days, a rampart 400 miles long encrusted with mine fields and artillery positions. The wall contains and protects the territory's ''useful triangle,'' where Morocco maintains its disputed dominance of its southern neighbor. Beyond the wall lies the bulk of the former Spanish territory that is claimed by the Polisario guerrillas, who have not yet been able to breach the defensive line. So, for over a year since the wall was built, there has been a stalemate in the military campaign, and the battle has shifted from desert confrontation to diplomatic jousting far from the territory.

President Reagan's special envoy to Central America, Richard B. Stone, arrived here this morning for a 24-hour visit and met with the head of the Government and his Foreign Minister. No official statements were issued about Mr. Stone's meetings with the Government leader, Daniel Ortega Saavedra, and the Foreign Minister, the Rev. Miguel D'Escoto Brockman. But it was understood that Mr. Stone heard complaints about United States assistance to anti-Government rebels fighting along the country's northern and southern borders. A source close to Mr. Stone's group said he had replied by reiterating President Reagan's suggestions that the Nicaraguan Government reduce the number of foreign military advisers in the country, halt aid to leftist guerrillas in El Salvador and negotiate with its adversaries.

Prime Minister Bob Hawke of Australia arrives here Saturday for talks with President Reagan and members of his Administration.