I can't understand why you did that." That's clear from the raised eye-brows, the shaking head, the moderate pace of the words.

"I can't understand why you did that!" Now the speaker's anger smashes through in harsh, fast, jerky movement.

"I can't understand why you did that." A child is talking now, mystification plain from wide eyes and slack mouth.

"I can't understand why you did that." Now the speaker is Marlon Brando in his role of Terry Malloy, with a slouch and mumble that proclaim, "And furthermore, I don't care."

Actually, no one is speaking at all. Not a word has passed anyone's lips since Bernard Bragg started showing a visitor the amazing capacity of sign language to convey what we hearing people think of as "tone of voice."

Bernard Bragg is a world-famous American actor you've probably never heard of. More precisely, Bragg is the leading deaf theatrical performer in America, the man who invented theater as a professional career for the deaf. In his sunny office at Gallaudet College, where's he's working on establishing a mime program for young deaf actors, his fingers darts, glide, jab, flutter as he chats about himself, his work, and deaf theater.

If the whole idea of a deaf actor seems odd, it's because you haven't seen him perform. As with any fine artist, to appreciate Bragg you've got to meet him on his own ground, which is silence-the fluid, highly charged silence of deaf people who communicate mainly in the language of signs. Within this language-and it is a real language, with its own cohesive rules, syntax and semantics, its own world of idiom and expression-Bragg, like all good actors, works to expand his everyday talk into an esthetic statement. Combining sign language with the separate medium of mime, he works in an amalgam that passes the traditional boundaries of either. He makes stories, poems, characters startlingly alive and at the same time newly mysterious.

The stage might seem an odd ambition for a boy born deaf, but Bragg says it was always his only goal. "I was practically born into the theater," he says. Both his parents were deaf, and his father founded a deaf amateur theatrical group when no deaf professional theater existed. At the New York School for the Deaf, Bragg led his class, and later he wrote poetry in college, but always, always, acting came first. He entertained family and friends with impressions of boyhood screen idols. He played Scrooge, among other roles, in high school and headlined in sign language productions at Gallaudet in the early 1950s.

But the stage offered no career prospects for a young deaf man when he graduated in 1952. Teaching was the livelihood that beckoned, and he moved to Berkeley and a job at the California School for the Deaf. The next few years brought mainly frustration as Bragg taught school and worked on a master's degree at San Francisco State, but mostly tried to find a direction for his life. Then suddenly in 1956, his path opened before him.

"I saw it in the newspaper," he says. "Marcel Marceau was performing in San Francisco." Bragg had never seen the great mime but had long heard of his artistry in the theater of silence. "I drove to San Francisco as fast as I could, and went to the theater. I sat in the balcony so I could watch the audience reaction." That a whole theater of hearing people could find such pleasure in a silent performer filled him with excitement and hope. "I had to talk to Marceau. I went backstage right after the show." In a scribbled note he introduced himself as young deaf schoolteacher and aspiring mime. Where, he asked, could he go for training? Intrigued, Marceau suggested he return the next day with a sample of his work.

For this unexpected audition Bragg perfomed two original sketches: Noah's Ark, with himself as both the biblical zookeeper and the animal passengers, and the One-Man Orchestra, playing all the instruments in turn while directing wtih his eyebrows. Marceau quickly solved the problem of finding a teacher. "Come to Paris this summer," he said, "and study with me."

Bragg spent a month under Marceau's tutelage, but through a series of coincidences ended his European stay as a consultant on a film about a deaf girl that starred Joan Crawford.

When the magical summer was over, he returned to California and a new double life-schoolteacher by day, entertainer by night, playing theaters, clubs like the hungry i, and European television. In 1967 he became a founding member of the National Theater of the Deaf.

His work has carried him across the country and around the world, showing hearing people everywhere the unexpected riches of sign and giving many deaf people a new sense of pride and possibility. Other activities, such as serving as technical adviser for the recent CBS-TV movie "And Your Name is Jonah," also combine these goals.

But in a sense he's never left teaching. "Non-handicapped people sorely need more and better information so they can dispense with fear, pity and false stereotypes," he says. "For instance, people still ask me: 'Do you read braille?"

Sign-mime opens to hearing people artistic vistas they never dreamed of. The movements are based on signs, but are "enriched for theatrical viewing." For those who don't known sign, the form is stunning in its fluency and visual surprise. For those who do, it is as resonant and evocative as poetry. But whoever the viewer, sign-mime-if successful-"sings to the eye." CAPTION: Pictures 1 through 4, No Caption, Photographs by Bill Snead