Behind Britain's military gains in the Falkland Islands are two factors that have attracted much less attention than the daily scorecard of planes and ships lost and damaged.

One is the Argentine loss of experienced pilots, a factor that U.S. officials believe is more important at this stage than the loss of planes.

Another, according to some U.S. officials, is that the British probably have commandos operating outside airfields on the Argentine mainland to warn the fleet when fighter-bombers are on their way.

These are among several elements U.S. specialists cite in explaining the military advantage that British forces seem to have gained in recent days in their battle to retake the islands, which lie 350 miles off the Argentine coast.

Unless Argentine ground forces garrisoned around Stanley on East Falkland counterattack soon against the British beachhead some 50 miles to the west, it is widely assumed here that the British successes will continue. Many U.S. specialists doubt such a counterattack is likely.

Officials warn, however, that the Argentine air arm still has the ability to inflict damage on the Royal Navy, which has already lost three major warships--one destroyer and two frigates--and had at least one other frigate severely damaged and towed out of the battle zone. More heavy Argentine air attacks and damage to British vessels were reported last night.

The British brought fewer troops to the Falklands than Argentina had there. At the outset, the British air arm of about 40 Harrier jump jets was also badly outnumbered, by some 200 Argentine warplanes on the mainland. But the training, tactics and technology of the British forces appear to be determining the outcome.

Although Argentina's Navy commander was said to be the most aggressive member of the ruling junta before the British invaded, the Argentine Navy--including one aircraft carrier and some dozen destroyers and frigates--has stayed close to the mainland and out of the fighting. It has been bottled up there by British nuclear-powered submarines, and officials here believe the Argentines are holding back their fleet, which is not very modern, because they are afraid to lose it and probably would.

The Argentines, sources here say, have virtually no anti-submarine warfare capability.

The Argentines have two good German-built diesel-powered submarines. These are handicapped, however, because when they go fast they make lots of noise and the British, who are good at anti-submarine warfare, could probably find and destroy them.

Thus the battle has been left to the Argentine air force, which as of Monday had lost an estimated 33 of its main striking force of some 115 U.S.-built A4 Skyhawk jets, French-built Mirage IIIs and Israeli-modified Mirages known as Daggers. But senior U.S. officials say it is easier to replace airplanes than trained pilots, and that is the crucial factor.

Many of the Argentine pilots were trained in the United States. As air force pilots, they are trained mostly for land attacks rather than attacks against ships. They are operating mostly from bases in southern Argentina, where the weather is bad, and flying to the Falklands, where the weather is also frequently bad. When they arrive, they usually have only enough fuel for one pass at the British fleet protecting the beachhead.

Neither the A4 nor the Mirage has radar that can help pinpoint surface targets, so the pilots must pick out their targets visually. This is a major problem in bad weather, and has kept Buenos Aires from launching strikes at night. Argentina is also said to have "very little" reconnaissance information about British ship positions before launching attacks.

Most importantly, the Argentine pilots are trying to knock out the British ships with bombs and short-range rockets instead of missiles, a very difficult job under any circumstance and especially with ships that are maneuvering and are heavily armed with antiaircraft missiles.

The Buenos Aires air arm, with one exception, is not equipped with the kind of "smart" guided missiles that can sink a ship from many miles away. U.S. experts say the Argentine pilots have done rather well, considering the fact that they are using bombs and are subject to withering antiaircraft gun and missile fire.

One official here said, "They are more courageous than cagey," meaning that the pilots might have done even better had they varied their tactics rather than come in in rather predictable waves.

The one exception to the bomb and rocket attacks has been the destruction May 4 of the British destroyer Sheffield by a French-built Exocet missile fired from a French-built Super Etendard fighter-bomber some 20 miles away. The Argentines have five Super Etendards and probably just a few Exocets. These weapons have not been heard from for the last three weeks, and officials here believe Buenos Aires is keeping them safe until they can use them for a particularly good shot at British vessels. The Super Etendard is the best plane Argentina has and also has usable radar.

The Sheffield was hit in the open seas. The British ships protecting the beachhead are operating in the 15-mile-wide channel between the two Falkland islands. While this restricts their maneuvering, it also makes it hard for Buenos Aires to use the Exocet, which should be launched from longer distances to be effective and to protect the pilot and plane.

The British appear to have used missiles effectively, although the loss of even modest numbers of warships to planes attacking with bombs will probably keep open the argument about whether surface ships can be adequately protected.

British-built Sea Wolf and Sea Dart missiles fired from ships are credited with kills against Argentine warplanes, as is the ground-launched Rapier missile. Two U.S.-built missiles in service with the British, the new ground-launched Stinger missile and the veteran Sidewinder air-to-air missile carried by the Harrier jump jets, are also said to have registered hits.

Sources here say the British are laying down steel landing tracks on the Falklands, which will enable some Harriers to move from carriers offshore to the island. Although U.S. officials say that neither side has clear air superiority now, moving some Harriers to the island would give Britain better air defense over the beachhead and allow some ships in the channel to withdraw to safer areas.

While the Argentines may have trouble finding the British fleet, sources here say the British "know when Argentine strikes are launched and the azimuth," or the direction they are coming from. This comes, they suggest, from the probability that British commandos were secretly slipped into Argentina to observe air bases and radio information back to the fleet, and from "access to other intelligence."

The mysterious crash in Chile last week of a British military helicopter is believed to have been a sign of the commando activity.