THERE ARE two faces to Deborah Tannen, and although they are like night and day, in her they are not incongruous. She is both serious and funny, intellectual and popular, aloof observer and intense participant. She is the rare linguist who can parse utterances in precise detail, then step back and draw penetrating conclusions about how language shapes our culture. By far the most commercially successful of professional linguists, she has written books that sell into the millions. Her latest, about men, women and conversation, You Just Don't Understand, has maintained a place on bestseller lists since it was published in 1990. It is, as Washington Post writer Henry Allen puts it, a book for the '90s if only because, for as long as the decade has been with us, it continues to prove it has hit a nerve: It virtually reels out of the bookstores.

Tannen was born in Brooklyn in 1945. She was the youngest of three sisters -- an important fact, she says, because alongside sisters she grew up believing there was no limit to what she could do. "I have no doubt that if I had had a brother, things would have been different. If a girl has a brother, she often gets the impression that he is allowed and able to do things that are off-limits to her."

Her father was a cutter in a factory in New York's garment district. Although he held a master's degree in law, times were hard and he had to take whatever work he could get: parole officer, treasury agent in pursuit of bootleggers, workmen's compensation lawyer, fabric man. Her mother was an electrologist -- a technician who removes facial and body hair -- taking clients into their busy Flatbush house during the day.

"My mother and father were great talkers," Tannen says. "My father would talk about his childhood and his work. My mother would fill us in on the lives of her clients. There was always a big conversation going on. Being the youngest, I was always sent to bed first, and I hated that. Nothing was worse than being told to go to sleep when all that good talking was going on."

She suffered a severe case of the mumps as a child, which left her hard of hearing. Today she wears hearing aids in noisy places and wonders if her interest in language cues wasn't spawned by the fact that she has always had to work to understand what is being said. At Hunter College High School, she learned to enjoy the intellectual life. "I had never been particularly socially adept. I was an outsider. But everyone at Hunter was a socially inept outsider. I felt at home, free to be me."

Tannen calls her days at Harpur College (now SUNY-Binghamton) her beatnik years. She dedicated herself only to the courses that interested her, ignoring the rest -- "getting A's and D's" -- and running off to every peace march and civil rights demonstration she could find.

After getting her B.A. in English in 1966, she set out for Europe. She was plotting a route around the world, but ended up in Crete instead, where she met the man she would marry. She stayed for a year, teaching English as a second language. There was a trip home to join the Peace Corps (which never happened because her parents so strenuously objected), then a return to Crete and a wedding. Eventually, however, her Greek marriage dissolved and she aimed for Detroit and a master's degree in English at Wayne State University. She taught remedial college English for four years in the New York area after that, but decided to further her graduate studies once her divorce was final. A summer linguistics institute at the University of Michigan caught her interest. It was there, at the age of 30, that she encountered the discipline that would change her life. "I was immediately hooked by Robin Lakoff's groundbreaking work in communicative style," Tannen says, and before long Tannen was pursuing her doctorate in Lakoff's classes at the University of California, Berkeley.

Since 1979 she has taught at Georgetown University, where she is one of three University Professors. She has also since remarried; her husband is Michael Macovski, an English professor at Fordham University.

Now the author of 14 books -- including That's Not What I Meant! (1986) -- at work on a 15th, and sought after by academic and popular audiences alike, Tannen still does research the hard way: taping, transcribing, listening, playing her material back to speakers and outside listeners for their reactions, spending long hours analyzing speech patterns. If her goal has been to bring linguistics to the common man and woman, she has certainly achieved it, but she has never sacrificed her academic rigor in the process. And if it is true, as Jonathan Swift once wrote, that "Nature, which gave us two eyes to see and two eyes to hear, has given us but one tongue to speak," it is not surprising that the two faces of Deborah Tannen -- the elfin and the sober, the popularizer and the academic -- speak in one voice when she explains why we say the things we do.