For three years, Mariana Bercovits Greenstein attended classes at the Georgetown University Law Center. She bought the casebooks, read the law and took notes.

In 1978, after her second year at Georgetown, Greenstein worked as a summer associate at the prestigious Washington law firm of Arnold and Porter.

No one, not even her husband, knew that Greenstein, 25, never applied to the Georgetown law school, that she never was admitted, paid no tuition, received no academic credit and took no exams.

It was no the first time that Greenstein created for herself the image of a successful young student.

The yearbook for the class of 1976 at Bernard M. Baruch College of the City University of New York lists Mariana Bercovits with highest honors as a psychology student, a college information officer said.

At commencement ceremonies at Carnegie Hall on June 10, 1976, she gave a brief address on behalf of Baruch students - including herself - who received service awards for contributions to the senior class, the information officer said.

School records show, however, that Greenstein never graduated from Baruch. She was registered as a student in the spring and fall semesters of 1973 but did not enroll again, officials said.

Mariana Greenstein's extraordinary life as a college graduate and law student began to come apart just a few days before commencement ceremonies at Georgetown Law last month.

When the list of graduating students was posted, a friend of Greenstein's asked school officials why her name was missing, a source said. Then a school official remembered than an out-of-town law firm, possibly checking a job application, had telephoned to ask if Greenstein was a student of Georgetown.

Georgetown's dean, David McCarthy, said in a statement after a Washington Post inquiry that, when Greenstein's case came to the attention of school officials, "an investigation was immediately instituted and the District of Columbia Bar authorities were notified.

"Thereafter, with the full cooperation of Ms. Greenstein, all pending applications for employment have been withdrawn and appropriate notifications to others have been made," the statement said. McCarthy declined to comment further about the case.

"It was a mistake. I knew it was a mistake. I just did not know how to emerge out of it," Greenstein said in a telephone interview.

"I wanted to be an attorney . . . I still do," she said. "It was a personal mistake.I didn't hurt anyone but myself"

G. Duane Veith, the managing partner at Arnold and Porter, confirmed that Greenstein worked for the firm in the summer of 1978 and that the firm understood she had completed her second year at the Georgetown University Law Center.

Veith declined to comment further on Arnold and Porter's hiring policies for summer associates. Generally, law students hired by the firm rank in the top 10 to 15 percent of their class and often attend the country's most respected law schools.

Knowledgeable sources said that large law firms like Arnold and Porter often hire summer associates on the basis of resumes and interviews without asking for transcripts, recommendations or proof of attendance at an accredited law school.

"Lawyers just think the badge of the profession is honesty," one source said.

"It's a system of trust," this source said. "There is no check, no certification. Nobody demands a thing."

Summer associates perform a variety of duties for the firms, ranging from research to advising clients on legal matters. Summer associates at Arnold and Porter in 1978 earned about $425 per week.

Greenstein declined to discuss how she got a job at Arnold and Porter except to say she went "through the employment process."

Sources said it was understood at the firm that Greenstein had done well at Georgetown. The quality of her work, however, was not up to Arnold and Porter's standards, sources said, and she was not offered a full-time job after law school graduation.

"We came to an understanding that I was not returning," Greenstein said.

Various sources said that Greenstein was seen at Georgetown's graduation, had registered to take the D.C. Bar and had applied to numerous law firms for jobs.

Greenstein said she does not have an application pending for the D.C. Bar. "I was not going to do more than I had already done" as a student, she said. Greenstein said she had not accepted a position with a firm, but she would not disclose whether any job offers had been made.

Greenstein said she did not graduate from Baruch because she didn't take care of "an administrative problem" that came up during her undergraduate years. Time passed, she said, but the problem - which she would not describe - remained, and her way of life "snowballed."

Dr. Jay Finkleman, the dean of students at Baruch and a psychology professor there in 1976, said yesterday that "my impression of her (Greenstein) is as a very outstanding student" who was well liked and active in a wide variety of school programs. He confirmed, however, that she did not graduate from the college.

Finkelman and the college information officer agreed that at a school as large as Baruch where about 3,000 students graduated in 1976 - a person could regularly attend classes and participate in student activities without being enrolled and without the knowledge of school officials. At the time Greenstein attended Baruch, tuition was free except for a small student fee. Finkleman said.

"I don't find it incredible because she (Greenstein) apparently established herself very credibly with people at the university while not being formally enrolled," the collegehs information officer said.

"You know we're not operating a police state," he said when asked about checks on student registration.

"It really was just a bad mistake. There's not other word for it," Greenstein said of the years she spent pretending she had achieved what she had hoped for as a career.

"It wasn't true on paper. It wasn't true officially," she said, "but this was not something I was trying to put over on anyone."

"Had I been doing it properly, I would have gone to law shcool," Greenstein said. She said she wants to start again - in college - and she hasn't lost her desire to become a lawyer. CAPTION: Picture 1, MARIANA B. GREENSTEIN . . . "It was a personal mistake"; Picture 2, Baruch College's 1976 yearbook lists Mariana B. Greenstein with highest honors. AP