A YEAR AGO, it seemed only a matter of time until Communists entered Italy's cabinet and took part, formally and explictly, in governing the country. But the Communists' plans have gone badly awary. The government in Rome has fallen again and, this time, its is an event of more than routine significance. It is a signal that the intricate system of understandings between Italy's two major parties -- the Communists and the ruling Christian Democrats -- has come unraveled.

The troubles of the Italian Communists are part of a larger pattern. In retrospect, its is clear that Euro-communism reached a peak of influence about 18 months ago, and has been losing ground steadily ever since. In the summer of 1977, the negotiations to bring the Communists into the Italian government seemed to be on track. In France, the parties of the unified left were riding high in the polls with a national election diretly ahead. In Spain, for the first time in a generation, the Communist Party was operating legally; its leadership turned out to be sophisticated and articulate.

But then the French left fell into a ferocious internal quarrel and, united no longer, lost the election last March. The Spanish Communists had to lie low last year to avoid jeopardizing the new democratic onstitution in the referendum last December. As for the Italiam Communists, the turning point for them was the murder last spring of Aldo Moro, the Christian Democrat who was the key figure in his party's slowly evolving relationship with them. In the regional elections shortly after the murder, the Communists lost heavily.

Since then the Italian party has been damaged by grinding friction between top and bottom. The leaders stuck with the strategy of supporting the rather conservative policies of a minority Christian Democratic government, in hopes of winning support in the middle classes and eventually climbing into the government. But the followers have increasingly demanded a return to the traditional line of radical oppostion.Last month they finally won. The Communist leazdership decided at last that the policy of cooperation was getting prohibitively expensive to them, and abandoned it. Since the minority government could not survive without tacit Communist support, it fell. That leaves Italy with no stable government, and no very clear prospect of one.

The next step hinges on an entirely new phenomenon, the direct election of a European Parliament next June. Both of the major Italian parties suspect that voters will behave differently in the European election than they do in their own national elections.Both also suspect that the differences will work to the advantage of the smaller parties in June, at the expense of the Communists. That gives the Christan Democrats every incentive to delay a national election until after June. The important thing here is not the stately and polite game of procrastination now being played in Rome. It is, instead, the dimensions that the European election is beginning to assume in the member nations' poilitics.

But how is the country to govern itself in the meantime, with with no visible majority in prliament? Perhaps something can be patched together for appearances' sake, but hardly anyone other than the professional politicians will greatly care. Constricted and ingrown, the Italian parliamentary tradition seems less and less relevant to the necessities of a modern industrial state.Italians seem to be looking increasingly explicityly to the European Ommunity for governance in the issues that matter.