The Reagan administration's expected decision to certify that El Salvador has fulfilled the requirements set by Congress to receive continued U.S. aid is based on a mixed record affected by the changing military and political situation in this war-wracked country.

Large numbers of civilians continue to be killed in the civil war, although observers agree that the death toll has declined during the past year. The U.S.-backed government's Army is visibly fractured and the top commanders are weak.

The main Washington-promoted phase of land redistribution will probably die March 3 after 50,000 provisional land titles have been given to peasants and numerous cooperatives formed.

The trials of soldiers accused in the murder of U.S. citizens are mired in appeals and counterappeals while a prime suspect has been returned to active duty.

According to U.S. officials here, the certification, expected to be announced Friday, could be made on the basis of such a mixed record because the language of the congressional requirement is relatively vague and because the nature of the war has changed dramatically during the past two years.

The U.S. Embassy's comparison of death tolls compiled by five different sources, including the rightist local press, the Roman Catholic Church and the leftist Human Rights Commission of El Salvador, show that the number of recorded "deaths attributable to political violence" declined considerably from 1981 to 1982. The embassy calculates a drop from 5,331 to 2,722.

Statistics from the Legal Protection Office of the Catholic Archdiocese in San Salvador show a decline from what was probably an exaggerated figure of more than 13,000 in 1981, before the church human rights offices were purged of partisan activists, to slightly more than 5,000 in 1982. But as a worker at the church office said last week, looking at grisly photographs of decapitated bodies recently found in some parts of the country, "It's true that the figures show a decline, but the captures go on in the same way, there are the same kinds of murders, there is the same will to exterminate and there are the same laws."

One foreign official here concerned with trying to get accurate statistical reports on the political violence noted that many of the likely victims of right-wing death squads blamed for many of the murders have either left for the hills, fled the country or been killed already. A few are in jail. Whole labor and professional associations with leftist connections were, as the official put it, "decimated" by the death squads in 1980 and the early part of 1981.

Until the guerrillas' January 1981 "final offensive," the Salvadoran civil war was based mainly on political confrontation, terrorism and intimidation rather than open combat. The death squads tied to the government fought "communist subversion" by killing anyone suspected of guerrilla sympathies, often in grotesquely imaginative ways.

The body count mounted as government forces tried to eliminate the "mass organizations" the insurgents had organized here. Corpses were literally strewn in the streets, in garbage dumps and beneath scenic overlooks.

Guerrilla military operations were limited to the occasional ambush and more frequent kidnapings and assassinations. But in 1981 the insurgents started taking on the Army directly. The war essentially moved out of the city and into the countryside. The death squad toll started a slow decline as there were fewer suspected "subversives" to eliminate in the city. Recently some of the most conspicuous body dumps, such as El Playon on the slopes of the San Salvador volcano have been receiving few victims.

But at the same time, civilian death tolls--attributed to the government and the guerrillas--have mounted over the last year in what the government now calls the "conflicted areas" in northern and eastern El Salvador. There is reason to believe that many of the Army's "guerrilla kills" are noncombatants.

U.S. Ambassador Deane R. Hinton cites what he calls an "interesting correlation" between the intensity of the fighting in the field and statistics on deaths attributed to political violence. In an interview earlier this week, Hinton noted that the number of captured arms reported by the Army rarely coincides with the number of reported enemy dead.

One aspect of the embassy's statistical study for the certification shows that if the Army's claims for guerrilla casualties are added to the compilation of press reports on individual incidents of civilian deaths, then the total number of dead begins to approximate the current church statistics.

One usually hard-nosed military analyst here said he does not believe that, as the Army claims, there have been "4,000 hard-core guerrillas" killed or wounded since March. In one operation in San Vicente Province less than three weeks after the last certification in July, the military claimed a "score" of 256 dead guerrillas as against three dead soldiers, and the analyst said he was told some of the dead were noncombatants.

For the moment, and for the purposes of certification, such deaths are lost in what Hinton called "the murky land of what the data show and what they don't show."

Closely related to the question of what the certification law terms "the indiscriminate torture and murder of Salvadoran citizens" is the question of who controls the Salvadoran armed forces and how.

Since the mutiny of a top field commander in Cabanas province earlier this month, the authority of El Salvador's defense minister, Jose Guillermo Garcia, is open to doubt. The officers in the Cabanas garrison represented a sort of "dirty dozen" of the Salvadoran Army that garnered at least tacit support from a wide variety of their comrades around the country, according to Salvadoran civilian officials.

And, while some military and civilian officials sought to portray the mutiny of Lt. Col. Sigifredo Ochoa as directed by right-wing political forces opposed to Garcia, Ochoa's second in command and one of his influential captains were long associated with the left-wing "military youth" movement.

Garcia now is expected to resign within a few weeks and a top Salvadoran official described his potential replacements as "less than enthusiastic" about some U.S.-promoted policies and reforms.

On the question of land reform, which the United States since 1979 has hoped would break the back of the ultra-rightist economic elite and undermine the popular support of the guerrillas, the report is mixed. The first phase of a three-part program announced in the spring of 1980 has been considered relatively successful in converting the country's largest private farms to peasant-run cooperatives owned, for the moment, mostly by the government.

The second phase of the program, and potentially the one that would have affected the greatest amount of land, would have expropriated and turned into cooperatives the country's medium-sized farms. It has never been put into effect.

The third phase, designed and heavily promoted by U.S. officials and agrarian reform experts associated with the AFL-CIO, was originally supposed to be pushed to completion in one year. By giving tenant farmers the right to buy the land they were renting in May 1980, it was supposed to benefit as many as 120,000 families.

But the so-called "land to the tiller" program had to be renewed twice before it was even seriously begun. It was almost killed by the rightist-dominated constituent assembly in June, and, although President Alvaro Magana and the head of the program have made dramatic efforts to push it along since then, the chances of the legislation being renewed in the constituent assembly after it expires in March appear highly unlikely.

In terms of the current certification, there is little question that during the past six months major advances have been made. The number of provisional land titles handed out to peasants has risen from only a few hundred per month during the summer to 5,000 every few weeks. A total of about 50,000 have now been issued, primarily in the more peaceful western part of the country, according to embassy statistics.

Julio Adolfo Rey Prendes, the Christian Democratic leader in the constituent assembly who would be expected to spearhead the fight to renew the law said late last week that the legislation is "a little ridiculous" in the context of right-wing opposition and war-related dislocation of potential beneficiaries.

On one of the most politically sensitive certification issues, the investigation and prosecutions in the December 1980 murders of four American churchwomen and the killings of two American agrarian reform advisers and the head of the Salvadoran Agrarian Reform Institute the following month, the Salvadoran government must show "good faith efforts" to pursue the cases.

The Salvadoran judicial system has proved extremely slow in the prosecutions, but U.S. officials believe that the process continues and bad faith has not been established.

The families of the churchwomen have pressed for investigations to discover the extent to which the murders were originally covered up and the possibility that the five Salvadoran National Guard enlisted men now under indictment were ordered by superiors to commit the crime. But there has been little movement in this direction and U.S. officials have said privately they have neither the concrete leads nor the desire to pursue such issues.

In November the judge in the case determined that there was sufficient evidence to convict the guardsmen and moved the procedure to what is called the "plenary stage" of the trial. Under the Salvadoran system, this means that the collected evidence is analyzed and debated in written briefs preceding the public hearing.

Defense lawyers have appealed the decision to advance the case and a ruling by superior court in San Vicente is currently being awaited.

The case of the agrarian reform advisers has also gone to plenary. There are two different appeals under way. One was filed by defense lawyers for two National Guard corporals who have confessed to being the triggermen. The other was filed by the Salvadoran attorney general's office in hopes of reversing an October decision that there was insufficient evidence to hold Army Lt. Rodolfo Lopez Sibrian in the case.

Both of the confessed triggermen say they were ordered to commit the crime by Lopez Sibrian, but the testimony of a confessed criminal cannot be used against an accomplice here.

Lopez Sibrian is now on active duty in Chalatenango Province.