TERI MANGAL, PAKISTAN -- In this primitive outpost a mile from the border with Afghanistan, the talk among young Afghan resistance fighters who cross the mountains to battle Soviet forces is mostly about something very modern -- the U.S.-supplied Stinger antiaircraft missile.

"The fighting is going much better this year," said one of the guerrillas of the mujaheddin, or resistance, who has been fighting for the past three years. The main reason, he said, is the arrival of the Stinger, a heat-seeking missile fired by ground troops. Informed sources here say the missile has about an 80 percent kill ratio against Soviet aircraft and helicopters.

"There are still very few of them here," said another guerrilla. "And only the most highly trained get to use them. We must get one plane per shot. So we use them against the higher flying jets and use rocket-propelled grenades and 12.5-mm machine guns against the low flying planes," he said through a translator.

"The Stinger has caused the Soviets to change strategy," adds a third resistance fighter. "Mostly they stay in their garrisons now. Their strategy is to hold on, since they don't have air cover all the time."

According to interviews here this week with resistance leaders, and with western and Pakistani military experts and diplomats elsewhere in the country, the overwhelming new factor in the eight-year war in Afghanistan is the impact of the Stinger since its arrival late last year. The Reagan administration also has dispatched the missiles to U.S.-backed insurgents fighting the Marxist government in Angola.

Reports from Washington that the Stinger may have fallen into Iranian hands through some of the resistance groups battling the Soviet-Afghan government forces are puzzling to officials here, who claim there is a good system of accountability for these weapons. They also point out that the missile is in the inventory of other friendly countries.

There is little doubt that the Stingers have knocked down very large numbers of Soviet-built jets and helicopter gunships, although some western military analysts here are skeptical of claims that about 450 Soviet and Afghan aircraft have been downed by all means since last October.

There is no doubt, however, about its impact on the morale of the mujaheddin. The missile, said one western official, has become something of a talisman for the resistance fighters, providing a sense of invulnerability for units that have them.

"Stinger is the magic word. It gives them confidence," said a senior Pakistani source closely involved in tracking the war.

"The mujaheddin have words for everything. But they haven't coined a word for that one. They like that word, Stinger," he said. Indeed, during conversations here in Pashto, a language spoken in Pakistan and Afghanistan, the word pops out in English.

The weapon's impact in curbing Soviet offensive tactics this year seems evident. But it is not clear how long its effectiveness will last or whether the Soviet and Soviet-backed Afghan government forces eventually will develop effective countermeasures.

When the missile first arrived inside Afghanistan, sources say it was used mostly in eastern areas, close to the Pakistani border, where, presumably, operations could be monitored more easily. Now the missile is in use in the northern battlefields as well.

These sources say the Soviet and Afghan air forces are flying virtually no missions in the north these days and are being extremely cautious in any area of the country where the missile is thought to be available. In those areas, Soviet and Afghan government forces have resorted mostly to artillery and rocket attacks against the mujaheddin.

Gulbeddin Hekmatyar, leader of one of the Islamic groups that make up the resistance, said during an interview in Peshawar that he believed the Soviets suffered such heavy losses in men and equipment in the past several months that they were not able to launch their regular spring and summer offensives.

"I don't see any prospect for a winter offensive," he added. "The situation is very much hopeful as we see it, very much better militarily than any year before."

The resistance fighters have been supplied clandestinely for years primarily by the United States. But they have also received help from China, Egypt and Saudi Arabia. Much of the weaponry in the rebel arsenal consists of conventional rocket-propelled grenades and machine guns that they have used, often ingeniously, to threaten some low flying Soviet and Afghan aircraft.

The resistance also deploys British-built Blowpipe antiaircraft missiles, which are less effective than the Stingers but nevertheless add to the overall problems faced by Soviet and Afghan flyers. The Blowpipe is optically guided, meaning the operator must keep the plane in his sights until the missile hits.

The Stinger, numerous sources say, has had the sharpest impact on Soviet tactics. The Soviets have diminished sharply their close air support of ground troops and use of helicopter gunships. Soviet bombing by Su25 jet attack planes now must be done at high altitudes where bombing is less accurate, sources say. The ability to strafe more accurately by flying slower also has been cut way back because of vulnerability to Stingers.

Similarly, informed sources say, the missile has reduced the Soviets' ability to carry out previously successful ambush tactics because those tactics normally involve the use of helicopters to airlift ground troops. That now also seems too dangerous. Most informants here believe the Soviets are trying to hold down their own casualties.

At this stage in the war, which began when the Soviet Union invaded Afghanistan in 1979, sources here say it is hard to assess the aims of Soviet strategy. There is very little offensive action, and some analysts believe the Soviets are deciding how to adjust their tactics to the more costly and complicated battlefield that now includes the Stinger. They also may be waiting until countermeasures against the missile are installed in their aircraft.

The mujaheddin have the first model of the Stinger. More advanced versions already have been developed in the United States.

Sources here say that the resistance fighters are able to use the weapons to achieve higher kill ratios than are achieved in U.S. Army testing. This is because the fighters use the Stinger as a very mobile weapon, infiltrating units to the edges of airfields to shoot down aircraft as they land or take off. Projected American use of the weapon is mostly for static defense against specific targets in the air, sources said.

Not all resistance units are equipped with Stingers, and morale may not be as high in those that do not have the weapon, one source said.

Similarly, while the resistance fighters inflicted some heavy losses on Soviet and Afghan government forces in May and June, the ground military operations of the mujaheddin still do not show much improvement in effectiveness, according to some observers here.

"What happens if you lose the Stinger edge?" asked one foreign official here. "There is still not much to fall back on."

Here in Teri Mangal, about 50 miles from the Afghan capital of Kabul, guerrillas who slip across the border for a few days of rest say their casualty rates are high, with probably five or 10 out of a 50-man group likely to be killed. They claim the Soviet ratio is at least twice as high.

Aside from bombing, the guerrillas say the greatest cause of casualties is from small antipersonnel mines scattered around the countryside by the Soviets. "We do not have enough experience yet finding them and handling them," says one guerrilla.

Despite what appears to be a relative improvement in the mujaheddin situation and a rise in Soviet military costs, "There is no sign inside Afghanistan to suggest a Soviet intention to withdraw," rebel leader Hekmatyar says. "They are building new barracks, consolidating their positions and bringing in fresh troops."