"Fair moment linger on," says Goethe's Faust as the moment flees. So it is with the euphoria Ronald Reagan brought to Washington last week.

For five days, a new era of good feeling had arrived. Flinty Scrooges exulted in the Reagan boom. Nuclear doomsday sayers discerned the rebirth of American power. Narrow partisans proclaimed a round-table administration reaching out to all comers. Folks off the farm rejoiced that a touch of class was being brought to the White House instead of blue jeans.

Sheer relief explained some of the glow. The welcome extended by the capital to the Reagans shows the other side of feelings long suppressed about the Carter period. In the first, symbolic steps toward the building of a new administration, furthermore, the incoming president has not put a foot wrong.

Harder choices lie ahead. Getting the right people in the right places in the Cabinet engages a large fraction of the future of the next administration. tOf central importance also are two major issues that tend to work in conflict with each other.

Inflation is the first. The core rate -- which is the rate businessmen have to raise prices from year to year just to stay even -- now approaches 10 percent. Bad harvests and uncertainty about oil supplies in the wake of the Iran-Iraq war threaten new bursts in the costs of food and energy.

One obvious way to scotch the increases is direct government pressure on wages and prices. But that approach runs counter to the national folklore, and has already been forsworn by the incoming administration.

Balanced federal budgets present another possibility. But the president-elect is pledged to a big tax cut next year, and international conditions require higher defense outlays. Deep cuts in social spending require legislation, and the chances are that the Republicans heading the Senate committees are going to be just as chicken as Democrats when it comes to paring Social Security and Medicare.

A tight money policy remains an option. But that would surely nip the present recovery in the bud. With plants idle, businessmen will be loathe to invest, thus causing productivity to lag. So a serious fight on inflation promises a long, long period of slow growth and relatively high unemployment.

Rebuilding American strength abroad, the second big problem, hardly presents a more fetching picture. The vortex of world politics now lies in the oil-producing states fringing the Persian Gulf. The Iran-Iraq war only provides new evidence that the local states cannot work out a stable balance among themselves. In the absense of such a balance, the Russians, already active in Afghanistan, Iran and Iraq, cannot be expected to sit on their hands.

American power is thus drawn inexorably on the scene. But this country does not now enjoy invulnerability in the matter of nuclear weapons. Neither does it have ready the conventional forces required for a stand in the area. In the absense of overwhelming American force, weak local regimes -- especially the one in Saudi Arabia -- show reluctance to commit themselves unreservedly to this country. The European allies, heavily dependent on oil from the area, find themselves sorely tempted to go into business for themselves with both the oil-producing Arab states and the Russians.

A slow, steady buildup promises the only way to arrest the decline of American power. Immediate attention needs to be paid to improving equipment and raising pay of forces already mustered. A long-term commitment to reduce the vulnerability of the strategic deterrent seems essential. Engaging the Russians in arms control negotiaions can -- on strictly security grounds -- be useful.

Striking a balance between these big problems is not impossible. It may turn out that emphasis on regaining strength abroad will foster the strong government needed to deal with the different economic problems at home. In any case, hope has a part to play in facing the future, and enthusiasm does not go amiss.

Still, seriousness and a sense of history also count. So does the capacity to distinguish between mere trivia and deep circumstance. The American Dream Machine constitutes a major obstacle to success in the rest of this bitter century. If there is an underlying reason for the decline in economic and international strength so many of us have sensed, it arises from the superficiality of American life. So it is mete to remember as the fair moment flees, that facts are better than fiction.