The principal man behind the State Department white paper on El Salvador was Jon D. Glassman, 37, a Foreign Service officer with a Ph.D. in Soviet studies and now a member of the department's policy planning staff.

The Washington Post had hoped to question Glassman in detail about the apparent mistranslations, ambiguities and errors in the white paper and attendant documents, but The Wall Street Journal had the same idea first and yesterday published quotations from an interview with Glassman.

Yesterday, Glassman instructed his secretary to tell a reporter that he could not talk to the press about anything.

In March, Glassman received unusual publicity for a Foreign Service officer when it became known that he had played a key role in finding and interpreting documents that led to the white paper. He was telling reporters then that he had never before seen such compelling conclusive documents as those he found in El Salvador. He also said emphatically that the documents not released with the original white paper only gave added credence to its conclusions.

But according to The Wall Street Journal, Glassman has since modified some of his views. The Journal's story attributed to Glassman the following acknowledgements:

That parts of the white paper possibly were "misleading" and "over-embellished," that analysts who drew up the white paper made "mistakes" and engaged in "guessing," that statistics about arms deliveries in the white paper were extrapolations and that analysts had attributed at least two of the documents to people who did not write them.

At yesterday's State Department briefing, Dean Fischer, assistant secretary of state for public affairs, read a statement from Glassman that said "the trust of the [Journal] article does not represent my views. . . ."

The statement said Glassman still considers the white paper "an accurate and honest description of the development of communist support for the Salvadoran insurgency." Glassman did not deny any of the specific quotations attributed to him by the Journal.

The Post sought to question other State Department officials yesterday about the white paper and supporting documents, but Fisher did not return a phone call until late in the day and then referred the matter to an assistant, who said the department woud prefer to read The Post's account before commenting.

Earlier, Fisher read a statement to the press in response to the Journal article, saying "we stand by the conclusions of the white paper" and adding that "the chronicle of events related in the white paper . . . are [sic] fully confirmed," without saying how or by whom.

The story of how the white paper was originally prepared has never been fully told. Middle-level officials in the State Department say they believe that it was pushed by the new secretary, Alexander M. Haig Jr., and that an important role reportedly was played by John Bushnell, a career diplomat who was acting assistant secretary for Latin America.

Glassman has told reporters about the crash effort to prepare the final document, an effort that involved several government agencies and many drafts. According to some accounts, the first versions of the white paper were less argumentative than the final one. Sources say those early drafts addressed the ambiguities in the evidence, a subject missing entirely from the white paper released Feb. 23.

The release of that white paper surprised some White House senior staffers. The same officials were even more startled two weeks later when Bushnell called in reporters to tell them that the press was paying too much attention to El Salvador -- attention that the white paper had appeared designed to attract.

The White House publicly rebuked Bushnell, and this incident contributed significantly to strained relations between Haig and senior assistants to President Reagan.