This time, the purists who insist that a new decade begins with the year ending with a 1 -- not a 0 -- have a point.This new year 1981, with a new administration and Congress gathering in Washington, represents a fresh start for the nation in a way that 1980 did not.

The mood is hardly buoyant, but it is realistic -- and there is a lot to be said for that. It is plainly going to be a time of hard choices, but that knowledge creates a climate where sensible debate may proceed without the disabilities of a dream world where all good things may be done at once.

The framework for that debate is well defined in "A National Agenda for the Eighties," the soon-to-be-issued report of a blue-ribbon commission named in 1979 by President Carter and headed by William J. McGill, the former president of Columbia University.

The introduction to the report notes that 20 years ago a similar commission named by President Eisenhower "reflected the optimism of an entire nation and a belief in the government's ability to address and solve its problems both at home and abroad. Throughout the decade of the Sixties, the nation's leaders expected that we could simultaneously eradicate poverty, go to the moon and win a war in Vietnam."

"Today as we enter the Eighties . . . we fully realize that the nation cannot proceed on all fronts at once. The nation faces a decade of difficult choices."

That is no news to the young David Stockman and the others struggling to frame Ronald Reagan's first budget, and it will soon be evident to Congress and the country. But the necessity for choice does not equate in any way to a policy of passivity for the national government.

On the contrary -- as both the ongoing budget analysis and the commission report make plain -- the one option that is not available to America is the continuation of the status quo. What is required is a searching reexamination of existing government programs and policies -- and of the relationship of government to the forces shaping the private economy.

In some areas, that will likely and properly lead to a reduction of the federal role. But in others, there may be new duties forced on the government.

Advance stories on the commission report, for example, have provoked controversy by questioning the wisdom of federal urban policies designed to slow the shift of population and industry from the declining cities of the Northeast to the growing cities of the Sun Belt. t

What has not been emphasized is that the commission calls for the creation of a national "minimum security income" program as a substitute for the welter of federal, state and local welfare programs -- a step that by itself might offer more fiscal relief to New York City than the mix of urban-aid programs.

The commission is right when it says that choices will have to be made in years ahead between "place-oriented" and "people-oriented" programs. And it is right, too, when it says that sorting out and choosing the right mixture of policies for the new decade is no task for the simple-minded.

"The answers to our dilemmas," the report notes, "do not lie in such slogans as 'less government,' any more than they lie in automatic dependence on federal solutions."

The decade now beginning can be a challenging and rewarding time for those involved in those choices. But the first step is the recognition that there is no escape from choosing.