Steven Seymour, who is paid $150 a day to help people understand each other, has converted President Carter's trip to Poland from the merely historic to the higher level of unforgettable.

Seymour is the man who stood before the microphone at Warsaw airport Thursday and carefully repeated, in Polish, Jimmy Carter's English-language remarks to the crowd.

There was just one problem - Steven Seymour got it wrong, badly wrong. Polish listeners alternately chuckled and blanched at what they were hearing and, in the aftermath, the United States was preparing an apology yesterday.

When President Carter told his welcomers that he had come "to learn you opinions and understand your desires for the future," Seymour missed the nuance.

"I desire the Poles carnally," Seymour spoke in Polish into the microphones.

When Carter referred to "leaving" the United States, Seymour had him departing "never to return."

Carter referred to the Polish constitution of 1791 as one of the three premier documents in the historic struggle for human rights.

Seymour plumbed that statement differently, having Carter say the Polish constitution is a subject for ridicule.

The hard day's night of Steven Seymour, according to Poles who knew better, was replete with other mistranslations.

Carter's "Pulaski County" came out as "Pulaski Duchy" His "won the admiration" emerged as "merited the respect." When he called Poland the "ancestral home of more than 6 million Americans." Seymour said it was "the fatherland of 10 million Americans."

Poles in the airport audience detected other linguistic booboos, saying Seymour used Russian syntax at some points and archaic Polish idioms that fade away a century ago.

Carter's hosts, after the initial mirth and shock, took the gaffes in stride, and polish Communist leader Edward Gierek was as forgiving and understanding as anyone else yesterday.

"No Pole would say a bad word about a lady or an interpreter even when we have to grit our teeth," Gierek told U.S. reporters, admitting that he had gritted his teeth.

Meanwhile, news services reported that Poland was awash with American jokes - as in, how many American does it take . . .? - and Steven Seymour, the butt of the humor, was out of his job.

After the big flap, Seymour was benched in favor of Jerzy Krycki, a Polish citizen who has worked for the U.S. Embassy in Warsaw. Krycki interpreted for Carter at his press conference yesterday and his latermeeting with Polish leaders.

Just before taking his place at Carter's side, Krycki confided, "I'm really scared."

Seymour, 36, is a professional freelance interpreter from New York City who has worked part time for the State Department since 1975, apparently winning praise for his fluency in Polish and Russian.

Donald Barnes, head of the department's interpreting branch, said that Seymour's previous work in trade negotiations had drawn critical raves and that he seemed to be just the man to handle the big show in Warsaw.

Barnes said that demand for Polish language specialists is so slight that the State Department employs no fulltime interpreters. For occasions such as Thursday's, the department hires free-lancers.

He said full reports had not come into Washington last evening, but he speculated that Seymour simply buckled under the "tremendous pressure . . . all those cameras, lights, people at the airport . . . It was a bit heavy."

One point still unclear was whether Seymour had attempted an extemporaneous translation of Carter's remarks or whether he had been given an advance copy of the statement for study. Seymour did not travel to Warsaw with the President, Barnes said.

"We felt he could handle this," Barnes said. "He seemed to be a logical choice. People had spoken well of his work. He had the skills."

Barnes said the Warsaw misunderstanding would not necessarily end Seymour's translating carrer at State, but he added, "I wouldn't sent him on another Polish assignment of this nature."

If things went badly for Steven Seymour in Poland, yesterday was no piece of cake for the man who sits on the State Department's Polish desk.

His name is Jack Seymour. When word of the language bobble circulated, Jack Seymour's phone began ringing off the hook.

"I am no relation to Steven Seymour. I've never met him. I have told my friends that all day. I tried to explain to an irate Polish-American lady who called me from Chicago to complain. She hung up on me." he said.