If President Carter hopes to utter anything immortal while in office - words to be carved in marble and into the memories of schoolchildren - he will have to think it up himself.

His five speech writers decline to make the attempt (and not only for the customary graceful reason that he would do it so much better than they).

As James Fallows and his staff see it, they have enough of a task to translate the ponderous workings and objectives of government into simple English. And this is probably more of a service to their boss and his constituents than turning out memorable prose.

"Our mission is to see that whatever Carter wants to say is said as clearly as possible," said Fallows. "Given that it's not Churchillian high rhetroic, or Sorensenian - I think it's more approporiate to the times. Fifteen years ago, the public responded to the graceful artifice of noble speech. People were less cynical."

What happened in between was that the public had a chance to contrast President Nixon's way of speaking, as documented on his office transcripts, with the way his speech writers had him speak in public and the speaking patterns of Presidents Kennedy and Johnson, as reported by people who knew them, with their speech writers' prose.

"Now it's more important to people to hear a rough approximation of how the person actually speaks, than to have something glamorous. It's reasuring to feel that you are seeing a slice of the real person."

Simple, declarative sentences come out of the speech writers' corner block of the Executive Office Building as they rarely do from other government offices.

"He doesn't want to get caught with anything that bears his name - Keven a routine proclamation - that can be ridiculed as gobbledeygook," said Griffin Smith. "His style is much simpler than it was a decade ago - he had to learn to express himself in public as he did in private," said Achsah Nesmith, who covered him as a Georgian politician for the Atlanta Constitution.

The speeches are unadorned with historical or literary allusion. Jerry Doolittle had a copy of Shakespeare's tragedies on his desk "because I'm reading 'Troilus and Cressida,'" not because he was finding something "appropriate." "His style is just like the New Yorker if you substitute 'I' for we' and leave out the affectations like spelling 'marvelous' with two L's," said Hendrik Hertzberg, who used to be one of the writers of the New Yorker's Talk of the Town.

Fallows, 27, a former writer for the Washington Monthly, Texas Monthly and Ralph Nader projects, is the chief speechwriter and the only one on the current staff who worked on Carter campaign speeches. Smith, 35, is a lawyer who also worked on the Texas Monthly. Nesmith, 37, had remained a friend of Carter's from her newspaper days and was a housewife in Philadelphia when the President asked her to join the staff. Hertzberg, 33, had written speeches for New York Gov. Hugh Carey in addition to his magazine work. Doolittle, 43, a former newspaperman and press attache, did press work in the Carter campaign. In addition, there is a researcher, Susan Battles.

Because there have been fewer than a dozen formal, written speeches given by the President, most of their work is preparing suggestions for informal remarks and re-writing proclamations, messages to Congress and foreign leaders, memoranda to heads of agencies and so on. Dulled by City Desks

Speeches are not generally group projects, but are assigned to one of the writers, who then works with the policy types, in the White House and elsewhere, concerned with the subject matter. After they agree on a draft, that is circulated, and the suggestions and additions of a wide variety of members of the administration are submitted with it to the President.

"It's dignifying the process to call it writing," said Doolittle. "Suppose you have Abraham Lindoln getting off the train with this envelope, and he sits down with his Cabinet. The Secretary of War says, 'You ought to say something about veterans' benefits. 'The Secretary of the Treasury says, 'We're going to have a balance of payments thing to deal with - the whole cotton market is shot to hell.' Pretty soon you're going to lose that fine, clear flow of prose.

"An idealistic young poet fresh out of Harvard might find this difficult - you have to have been dulled first by a lot of city desks," said Doolittle, who wrote for The Washington Post, The Star, The Daily News and The Northern Virginia Sun. "The end result may not be art, but you hope you convey a certain tone."

Nesmith defines the tone as "very low key - he never was a ranting, raving, traditional southern speaker who used politics as entertainment," and says Carter was able to develop his style because he never used a speech writer until he entered national politics.

"Like Talk of the Town, it's supposed to be a single human voice speaking in a relaxed manner," said Hertzberg, who contrasted it with Gov. Carey, "who is much more oratorica with more dependent clauses, longer sentences, longer paragraphs, a purpler sytle." Murky Prose

But the sharpest contrast is with the government prose which comes in, in what the speech writers regard as raw material to be re-written by them "into English."

"I can always tell which government department a statement comes from - each has its own particular murky style," said Smith. "From some of them, inflated words just keep coming out, like dough rising. It's fascinating to take some statement and pursue it all the way down the bureaucratic chain until at last you find a person who actually knows what it means and will translate it."

If they will agree to. "Often, they say, 'It sounds better your way, but you'll have to say it our way - our words have code meanings to a lot of people,'" said Doolittle. "And I say, 'Well, if it's written in code, why can't we translate it into English?'"

Fallows is fighting the battle against such government words as "modality" and "interface," the more widely mis-used "relate" and "hopefully," verbs made out of nouns, and several nouns lined up together" (the example he gave, "the Carter" backed fuel efficiency program," being from The Washington Post).

He translates the presidential "it caused me extreme gratification to learn . . ." into "I'm happy that . . ."

In the presidential letter to new citizens, the old opening. "Of very special importance is the fact that while many of us are citizens by birth, you have by choice selected America as your new land," was re-written to "You are an American by choice, and that is a source of special pride to me and to all your fellow citizens." 'Not This, But This'

He tries to avoid what he calls "the garbage can theory of throwing in everything - 'of particular importance,' 'high priority,' 'has issued guidance,' 'aggressively,' 'undertaking this effort,' 'growing trend.'"

"There's something about writing for a President that makes you inflate things" said Smith. "How easy it is to write as if everything were the most important thing in the words that connote urgency and drama. "The specter of world catastrophe" - what's going to happen if a President uses a phrase like that?"

Hertzberg said he particularly dislikes the "not this, but this" school of speech writing. Doolittle says he has "a certain amount of suspicion of the ringing phrase," but had to admire a former President "for the wonderful slogan. 'Paranoia for Peace.'"

Doolittle does the humorous lines in prepared speeches, but says the President is barred from "off-color jokes black humor, ethnic humor, put-down humor, jokes about political opponents, religious jokes, male-female jokes, racial jokes - which is why he tells a lot of stories on himself or Billy or a few close associates. If he pokes fun at Jody Powell, the next day people won't be running around town saying, 'Jesus, Jody's in trouble.' But if he told one on the Secretary of something or other, everbody would be saying, 'Oh, oh - old Harry's on the skids."

Carter himself has decreed one firm style rule - that masculine words not be used exclusively when men and women are meant. "No 'mankind' or 'the working man' - if we ever use 'he,' he says, 'Where's the she?'" said Fallows.

But then, when they've got everything perfect, to their satisfaction and the President's, he is apt to abandon their efforts and decide, instead, to tell one audience about his conversations earlier that day, with other people.

"He says what he wants to say - he would write it all himself, if he had the time," said Fallows. "That's something we never fool ourselves about. We're not personally responsible for words that echo down through the ages."