The young Russian now carrying his AKM assualt rifle on duty in the Afghanistan invasion that has shaken the world is a 19-year-old, unmarried Soviet draftee who earns about five rubles ($7.50) a month in the service of his country.

He was drafted after turning 18, spent a month refreshing high school-trained premilitary skills of shooting a rifle and saluting, then went straight to a combat unit to serve out his two- or sometimes three-year hitch.

He has been seen berfore in the post-World War ii era and each time his presence on foreign soil -- 1956 in Hungary and 1968 in Czechoslovakia -- has struck with the impact of an earthquake on governments around the globe. Even so, little is generally known about this Soviet man-at-arms beyond the basic facts -- he is well-trained, intensely patriotic son of working-class Soviet parents whose limited notions of the land he now occupies are shaped by the controlled Soviet media.

This rifleman, a composite drawn from authoritative sources and the official Soviet press, is a part of what Western military analysts regard as one of the most formidable fighting forces in the history of warfare. Probably not one in 500 of these same experts would have predicted just a few weeks ago that this soldier -- suppose his name is Ivan Gruznikov -- and his 90,000 or more comrades-in-arms would now be stationed in Afghanistan, ready to repel what Pvt. Gruznikov may have been told is an imminent American military invasion of a fraternal Socialist neighbor.

The reason the experts could not have predicted Gruznikov's sudden appearance -- via four-engined Antonov-12 transport planes, or by truck along the Soviet-built road that leads down to Kabul from the Soviet border -- is that his army, like the armies of the Western powers, is designed to win a conflict in Central Europe.

How this Soviet conscript came to be in Afghanistan and what that bleak fact may mean for the world has been written about exhaustively in the days since the Dec. 27 Soviet intervention, and will be scrutinized in months and years to come. Nevertheless, a look at the soldier and the fighting force in which he serves may provide clues to Soviet power and Russian life.

Gruznikov is not thought of in Soviet military doctrine as a "grunt," or foot soldier, but as a highly mobile rifleman. Most of the Russian fighting units operating in Afghanistan are called motorized rifles, highly mechanized divisions of up to 12,000 men when at full strength. They include up to 250 medium tanks, several dozen light tanks, more than 400 armored personnel carriers of various types, 100 field howitzers, anti-tank guns and hundreds of anti-tank missiles, formidable self-propelled armored anti-aircraft batteries, and four unguided Frog artillery rockets capable of sending a 1,000-pound warhead about 50 miles.

Almost 70 percent of the 1.8 million-man Soviet army -- 115 of 169 divisions -- is motorized rifles and about half of these are stationed in European Russia and Warsaw Pack countries, stiffened by 80 percent -- 38 of 46 divisions -- of all its tank forces.They are designed around Soviet doctrine which forsees a short war of motion and massed firepower against NATO countries for control of Central and Western Europe and to repulse a Western invasion into the industrialized population centers of Russia. Most of the rest of the army is deployed in 38 rifle and six tank divisions along the Sino-Soviet border, where armed clashes flared in 1969 on the Ussuri River in the Soviet Far East.

Unlike American military planners, the Soviets foresee war close to home. Then can depend upon much shorter supply lines and thus Gruznikov's rifle unit has far more fighters per supply personnel -- what experts call the "tooth-to-tail ratio" -- than a comparable American unit.

In the 11 years since the Soviet invasion of Czechoslovakia, the Soviets have compiled an impressived record of projecting Russian military might far from home. Such successful military airlifts as the 1975-76 Angola operating in support of Cuban troops, to Ethiopia, have recently to South Yemen have proved the global potency of Soviet power in the minds of Western strategists.

Moreover, the rifle unit to which Gruznikov belongs -- along with Soviet rocket, air and naval forces -- has been steadily upgraded with new weapons and related equipment, reflecting a continuing technological revolution within the Russian military establishment, which claims about 15 percent of the total Soviet gross national product annually, according to reliable Western estimates.

This has brought such things as electronic night vision aids, computerized fire control systems and other advanced hardware to Soviet battle units.

Yet Gruznikov remains largely as he has always been: a son of the soil, who wears World War i-style foot wrappings instead of socks into battle, is deeply loved by his parents and who will most probably not re-enlist once his active duty is finished.

This is because by almost any standards, his army life is grim. If married, family stays home; if engaged, no marriage while on active duty. As a conscript, he gets no regular leave: that is at the whim of his officers, though a death in the family or some similar event will get him some time away.

Barrack life is crowded and dominated by tough, non-commissioned officers a few years his senior and professionally educated junior officers churned out by the nation's estimated 143 officer schools to run this conscript army. If stationed in the East Bloc, as about 350,000 Soviet troops are, Gruznikov is virtually barred from casual contact with the natives.

Reveille comes early. Emphasis is on physical fitness, spartan diet, arms drills, discipline, and mastering his specialty.Gruznikov gets about five hours' political lecturing weekly, but his training is concentrated on realistic battlefield exercises in which he also gains experience operating without officer direction.

In the field, he can expect one hot meal a day, plus traditional bread and hot tea. It may not sound appetizing, but his tinned fish and soup or stew is enough "to keep a man fighting," as one source said. His basic weapon is his AKM Kalashnikov automatic rifle, produced in greater quantities than any other modern small arm and considered the world standard in infantry weapons. It fires 7.62mm slugs from a 30-shot clip, and Gruznikov carries about seven extra clips with him as well as a new bayonet able to cut human flesh or electrified barbed wire with equal ease.

Gruznikov's biggest personal problem is that, like his civilian counterparts, he drinks too much.

The coming decades promise more complex problems for military authorities. At present, Gruznikov is a Slav drawn from European Russia. Slavs make up about 68 percent of Soviet forces, but western demographers say the declining birthrate of the Slav, coupled with booming births in Central Asia, implies that by the year 2000, more than half the Red Army will be drawn from the Soviet Union's Asiatic peoples.