Each weekday at 6 a.m., Capt. Jesse Garcia, a 32-year-old Green Beret from Superior, Ariz., climbs into an armor-plated, 1976 Ford Maverick with bullet-proof windows and drives 10 miles from his suburban Guatemala City home to the Escuela Politecnica, the Guatemalan Army's officer training school.

There, he dons U.S. Army jungle fatigues and drills the cadets in the weaponry, tactics and strategy of antiguerrilla warfare.

During a recent four-hour interview at his home, and on a motorized patrol with 40 of his cadet students, Garcia described himself as a military trainer whose function is "not much different" from that of U.S. military advisers in neighboring El Salvador.

Now four months into a two-year tour of duty in Guatemala, he said he is authorized to teach cadets there "anything our Army has," and his subjects include training in ambushes, surveillance, combat arms, artillery, armor, patrolling, demolition and helicopter assault tactics.

Garcia seemed unaware of the existence of congressional restrictions that since 1977 have barred Guatemala from receiving any U.S. military assistance or training because of alleged human rights abuses. At the same time, he appeared not to know that the Pentagon officially describes his position, as it did that of another Green Beret officer who held the post during 1980-82, as that of a language instructor who teaches only English to Guatemalan officer candidates.

Asked about Garcia, Pentagon spokesman Maj. Carroll Williams said Garcia "may speak to his students on a variety of subjects, but his primary mission is not to teach tactics. He's an English teacher." Garcia, however, said in Guatemala that he has no language-instruction responsibility.

According to Rep. Michael Barnes (D-Md.), chairman of the House Inter-American affairs subcommittee, the activities Garcia described and demonstrated recently in Guatemala are in direct violation of a congressional understanding that "no military assistance including training can be provided for Guatemala without the approval" of Congress. "I hope and expect the reports . . . are untrue," he said.

According to one U.S. official familiar with the Pentagon's program, the idea of "carrying an adviser on the books as a language instructor" was developed jointly by the Pentagon and the State Department. The position in Guatemala was created in 1977, the year Guatemala first declined to request U.S. military assistance because of the human rights restrictions.

"Because of the heat in Congress, and the fact that it was easy enough to hide one person in that category," said the official, who asked that his name not be published, the trainer was sent under something called the Personnel Exchange Program. Dating back to the 1940s, the program is designed to foster inter-Army cooperation by sending U.S. officers overseas and bringing foreign officers to the United States.

The program involves little cost, with the officers' salaries and moving expenses accounted for under the Army's operations and maintenance fund, a Pentagon spokesman said. The Personnel Exchange Program does not appear as a specific line item in the defense budget and does not require congressional authorization.

The idea, said the official familiar with the program, was to "keep the door open with the Guatemalan military; to maintain some presence" with the Army-run government. The classification of the trainer as a language instructor, he said, was "a running joke."

Carter administration State Department officials questioned yesterday about the program said they had no direct knowledge of it, but pointed out that, despite its suspension of weapons aid to Guatemala, the administration had advocated a "foot in the door" strategy to maintain contacts and had requested a military training program that was rejected by Congress.

Barnes' subcommittee has remained largely at odds with the Reagan administration over the need and justification for resuming military aid to Guatemala.

The Reagan administration maintains that the human rights situation there has improved under the new government of Gen. Efrain Rios Montt, who took power in a coup last March. The administration has argued that the aid is needed to counter a growing threat from leftist guerrillas who for decades have been striving to overthrow a series of military governments in the country the State Department describes as the "real" target of Cuban intervention in Central America.

But amid persistent reports of Guatemalan Army massacres of civilians, including an Amnesty International charge that 2,600 have been killed since Rios Montt took over, and continuing controversy over U.S. involvement in Central America, Congress thus far has refused to approve any military program there.

Although the full House Foreign Affairs Committee recently authorized $250,000 requested by the administration to train Guatemalan officers at U.S. bases, committee sources said the bill is not expected to pass on the floor of the House. It makes no provision for placing U.S. trainers inside Guatemala.

Barnes said he had no knowledge of Capt. Garcia's presence in Guatemala, saying that "the involvement of a member of the U.S. military mission in training activities" there "would be completely contrary to assurances that Congress has repeatedly received from the Reagan administration that no such training is occurring."

According to the official familiar with the program, the Pentagon and State at various times have considered increasing the U.S. military presence in Guatemala under the program, but they did not do so for fear of discovery. Since Rios Montt's takeover, however, he said, a proposal to increase to five the number of trainers has been under consideration.

Green Beret Maj. Larry Salmon, Garcia's predecessor at the Politecnica, said the Pentagon considered sending more Personnel Exchange Program officers to Guatemala while he was on duty there. Reached by telephone at the Command and General Staff College at Ft. Leavenworth, Kan., where he is studying, Salmon said that during his tenure at the Politecnica, he "was basically an English teacher and helped them design a curriculum."

But Salmon, who taught unconventional warfare at Ft. Bragg before his Guatemala tour, acknowledged that he helped the Guatemalans plan their tactical training, accompanied cadets on patrols, instructed in parachuting at the Guatemalan Army's base at San Jose and gave brief instruction at the Politecnica on how to break down the M16 automatic rifle.

Garcia, who described himself and Salmon as "the same kind of officer," said their functions basically were the same.

Many of his cadet trainees, Garcia said, go directly upon graduation into combat.

"They just graduate and already they have a unit," he said. "If they're infantry, they'll go straight out to the mountains."

One of his teaching techniques, he said, involves taking overnight patrols into the hills around the Politecnica armed with Galil automatic rifles, fragmentation grenades and field howitzers.

"These guys could actually be in a firefight with real guerrillas" who use the hills as a staging area, said Garcia, who carries a .45 pistol which he said he is permitted to use only in self-defense.

Garcia said his instruction emphasizes patrolling methods in the countryside and in rural villages. He teaches reconnaissance, "direct action" and "destruction patrols." During a "direct action" patrol, he said, "you go to an objective and destroy it, raid it, harass it." On destruction patrols, he explained, the troops would use gunfire, artillery, explosives or aerial bombardment to "destroy a piece of equipment, destroy bridges, destroy towns."

He said the Guatemalans have a special interest in being trained in the use of U.S. weapons. Those in the Guatemala arsenal he expects to teach include the M16 and M1 rifles, the M79 grenade launcher, U.S. fragmentation grenades, a recoilless antitank rocket that Guatemalan officers said is used as an antipersonnel weapon, and the 60- and 81-mm mortars.

Garcia said he also expects to be giving airmobile combat instruction. He said that if the sale of spare aircraft parts requested by the administration is approved and the Guatemalan Army is able to field more helicopters, "they'd have more control over the population . . . . Machine-gun fire is pretty deadly from the air."

He said the Guatemalans are "very interested in Vietnam."

"I tell them that we had the firepower to win that war, but we weren't allowed to use the firepower that we had . . . . We weren't allowed politically," he added.

Asked if any restrictions are placed on the military techniques he is permitted to teach, Garcia replied: "Negative. The embassy doesn't tell me anything."