The trouble with being 80 years old is that everyone always wants you to talk about the past, when the thing you need most urgently to talk about is the present and, maybe even more, the future.

Old Washington Hand Richard L. Strout had his 80th birthday yesterday, and the people at the New Republic were waiting for him, standing in a ring around a table loaded with petit fours, strawberries and champagne.

Since 1943. Strout has been writing the TRB column, which appears inside the front cover every week. And that isn't even his main job. He's been a reporter for the Christian Sciene Monitor since 1921. Starting in Boston. One of those temporary appointments.

At last he appeared, and everybody clapped, and editor Martin Peretz connected him to a plastic glass of the better champagne, and he lounged in the door, tall and trim in his dark blue suit and drawled:

"This is my second surprise party today . . . I have one more to go."

(Before he got there, the phone rang, and because it had been arranged for Vice President Mondale to call in his congratulations, and New Republic's dean, John Osborne, took the call. The voice at the other end was singing "Happy Birthday," and Osborne kept trying to break in, "But Mr. Vice President - " until he realized he was being hoaxed by staffer Ken Bode from another room. He hung up with an oath, to general hilarity. Later, President Carter himself phoned "and had some very nice things to say.")

They gave Strout a pair of oval goal cuff links engraved "TRB." The initials, a house pseudonym, are supposed to be an editor's inspiration upon glancing at a New York BRT subway car. Evidentually the editor was standing on his head at the time.

Then the guest said a few words, which neatly summarized an interview earlier that morning, when he expressed his passionate concerns about this country.

"We have a clumsy form of government," he said then. "We deserve a better one. You see it in various ways: We elect a president first, and then find out who he is. There's been a national failure of nerve, of will power, over the energy problem. We can't seem to make ourselves do what we have to do. I don't want to be too pessimistic, but the last 30 years have seen a decline in American preeminence."

He quoted Alvin Hansen, one of his favorite professors at Harvard (class of '19), who in 1945 wrote confidently that Americans "can undersell any competition in autos, radios, typewriters, etc."

"Now look at us," Strout said. "In 1945 we couldn't believe we'd have all this foreign competition. We couldn't believe we could ever lose a war. Well, we've lost a war. We've had a faithless president . . ."

Ten years from now, he added, our Constitution will have its bicentennial, and maybe it's time we took a hard look at it. Do we really still need a guarantee that citizens won't have to quarter troops in their homes?

And this business of requiring a two-thirds vote on foreign treaties: "Can you imagine how I felt, coming back from fighting a war to make the world safe for democracy, to see the League of Nations lose out in Congress? At least three out of four senators wanted one sort of league or another, but they couldn't agree on which one, so it lost."

Maybe, he mused, we should study the Canadian parliamentary system.

He is regarding the Federalist Papers on treaties. He finds Jay and Hamilton, who wrote their pieces 190 years ago after all, poignantly out of touch with the needs of today's industrial nation and its global responsibilities.

"At the moment it seems that five people will decide the Panama treaty issue. The lobbying, the promises . . . it was Christmas yesterday on the Hill."

He has been a lot of lobbying in his 55 years in Washington. It used to be the farmers who had the most powerful lobby. He doesn't expect the technique to go away.

He worries about the seesaw of power between president and Congress, notes that when Congress grabs the reins it never seems to do much with them and soon gives way to the executive branch again. He recalls George Reedy's 1970 warning about the specter of a Man on Horseback.

"I don't feel we learned a thing from Watergate," he said.

He mourns the "dangerous decline" in party strength, the weakening of the two-party system, the disaffection of voters and especially the failure of blacks to organize their voting power as they might.

"Also, I see a failure in the national nerve on this question of illegal immigrants. Mexico City is on the way to becoming the biggest city in the world. We have a million illegal immigrants a year coming in, taking jobs - mainly from black people - and we don't know how to deal with it."

In fact, the whole problem of world population growth has bothered him for years. It is a theme that runs through many of his TRB columns like a vibrant bass note of doom.

Someone wanted to know how he kept that fine edge of outrage in the column, week after week.

"It's not so much anger," he said. "I'm a reformer, and I see injustice - well, what it really is, is a lack of imagination. Imagination is the difference between a liberal and a conservative."

This morning, free at last from the surprise parties, the presents on his desk, the interviews by other reporters, he would go back to work. Perhaps would lope over to the National Press Club for a Richard Strout Salad (cottage cheese), or eat a liverwurst sandwich at his roll-top desk. And would go home to the big house on Foxhall Road, so quiet now without the five children or the 10 grandchildren. A widower for eight years after his first wife died, he married again in 1939.

With a noon deadline, he says he doesn't get in as much legwork as he would like, but you'd never know that from his incisive, balanced and always readable copy.

"I've been rummaging through a lot of my old stories for a book of reminiscences I'm doing. And you know, I used to say we reporters write on water, but now I see that we write on paper. Which crumbles.