If the rules of economics operated as advertised, the United States would be suffering from inflation today.

But, obviously, the laws do not always work. One in particular - a rule that says inflation goes up when unemployment goes down - seems to have broken down. And that helps to explain why forecasts of higher prices have not come true.

The great expectation through the 1980's, enshrined in college economics textbooks, was this: Inflation would be triggered when unemployment fell below 6 percent, according to a formula known as the Phillips Curve. Last August, the 6 percent barrier was finally breached. But the inflation rate in the first quarter was actually lower than it was last year.

The breakdown of the Phillips Curve, developed by A.W. Phillips of the London School of Economics in the 1950's, does not mean the inflation rate may not soon start to rise from its present, modest annual pace of 3.9 percent. But if it does, it will be for reasons that have little to do with civilian unemployment, factors such as brisk consumer spending, the weak dollar and the export boom, hefty factors all, but none with the inflationary push of higher wages.

Wall Street and Washington now pay closer attention to such seldom-discussed guidelines as the employment cost index, average hourly earnings, unit labor costs and union settlements than to the unemployment rate for signs of rising wages. And all these Labor Department statistics show virtually no movement in the past year.

Of course, employment and inflation are not completely unconnected. No economist would go that far. The unemployment rate is now 5.6 percent, which it reached in February and March, the lowest level since 1979; the April figure, to be announced Friday, is expected to sustain the downward trend. If it doesn't stop sometime, the pool of unemployed workers will become so small that executives, in theory at least, will have to bid up wages to win away workers from other companies. Then, presumably, the executives will raise prices to cover their higher labor costs.

''But no one knows today what the unemployment level is that makes inflation happen,'' said Robert M. Solow of the Massachusetts Institute of Technology, who won last year's Nobel Memorial Prize in Economic Science. ''We should be cautiously moving the rate down to see what will happen.'' Such experimentation would show that the trigger point is between 4 and 6 percent unemployment, and probably closer to 6 than to 4, said Prof. Lawrence Summers of Harvard. Finding the Trigger Point

One beauty of the Phillips Curve was that a so-called trigger rate of unemployment could be calculated and displayed on a chart. The point on the chart just above the trigger was called the natural rate of unemployment: a level that neither pushed up wages and inflation nor pushed them down. This equilibrium rate was put at 6 percent by Paul Samuelson in the 12th edition of his classic textbook, ''Economics,'' published in 1985.

Until the early 1980's, wages and inflation were closely linked, as the textbooks taught, although cause and effect were often unclear. Sometimes rising wages triggered higher prices; other times, different factors, such as rising oil prices, increased the inflation rate, and then labor sought higher wages to keep pace. A Fading Practice

And with the Phillips Curve for support, policy-makers in the 1970's and early 1980's had sometimes taken steps to slow the economy, even risking recession, when the unemployment rate reached what everyone thought was the trigger point. When that last happened, in 1979, Paul Volcker, who was Federal Reserve Chairman, was developing his anti-inflation strategy. The practice ''is not completely dead today, but it is certainly a faded one,'' Mr. Summers said. ''The approach now is to keep testing.''

One test is the employment cost index, covering company outlays for wages, salaries and benefits. The Labor Department announced Tuesday that the wage element of the index for the private sector rose only 1 percent in the first quarter. Even in the Northeast, where the unemployment rate is below 4 percent, low enough to begin to draw people from sections of the country where unemployment is higher, wage increases had leveled off.

To be sure, wages there have risen faster than in other parts of the nation, in part because the pool of workers is tight. But even in New England, as in the rest of the country, labor's power to command higher wages is greatly diminished, for many reasons.

The best-known is management's success in convincing workers that American companies and the jobs they provide cannot survive against lower-cost foreign competitors - unless wages are held down. Unit labor costs - that is, the labor cost for each item produced -went up only 1.9 percent in 1987, rising much more slowly than prices. Average hourly earnings have hardly budged this year and, when adjusted for inflation, are lower today than they were a year ago.

Job security, in sum, has replaced higher wages as a top priority. And despite the low national unemployment rate, there has been a rise of roughly two million in the number of people who work part time, but would gladly work full time, according to the Labor Department.

In manufacturing, productivity has risen at a healthy 4 percent annual rate in recent years, which means workers are producing more in a day's work. But very little of the income from this greater production is being passed along as higher wages - a departure from the practice in other postwar decades. A growing number of companies now offer bonuses and other one-time payments in lieu of permanent increases in base pay. And many retailers, particularly in New England, with its low unemployment rate,, are understaffed rather than bid up wages enough to attract more workers.

Still, at 5.6 percent, the civilian unemployment rate is unusually low, perhaps only a notch away from the trigger point, says Audrey Freedman, a labor economist at the Conference Board, a business organization. ''But a recession might easily intervene,'' she said, ''and then the unemployment rate would rise again.''