YOU have to understand the culture if you are going to work here," says Kathleen Keith in a telephone interview. She is the only American in the advertising department of All Nippon Airways in Tokyo. "You have to know when to push and not to push. You have to be open-minded and flexible."Shortly after her arrival in Japan, Ms. Keith realized that although she was a manager handling international ad campaigns for the airline, her boss also expected her to make his tea. She did it for about three months until she felt confident that she could express herself well enough in Japanese not to offend, but to get her point across, firmly. She told her superior that serving him upset her so much it was beginning to interfere with her work. He accepted the explanation and relieved Keith of the tas k. Like many other American women working for Japanese companies in Japan, Keith has found herself in a situation filled with paradoxes. Attempts to modernize notwithstanding, Japan is a country where protocol and hierarchy continue to define social and career interactions. Men enter a company expecting to work hard and rise fast. Women are siphoned off into an "office lady" track and treated gingerly if they express long-term job goals. Opportunities for women to rise into management exist in an ever-changing Japan, but they are still very limited. Nearly 40 percent of the work force is now female, but most are in lowly jobs and leave after a few years. So what happens when an outspoken, ambitious young American woman, recruited right out of college with promises of subsidized housing, a good salary, and sogo shoku - a tenured position usually offered only to male "lifetime" employees - arrives? "I'm distinguished from the Japanese women as 'not cler- ical, Keith explains, "and I don't have to wear a uniform." A uniform? Men are permitted to dress as they please, says Keith, but women are required to bedeck themselves in ugly navy blue jumpers. This, it turns out, is true in most companies. The concept seems to be: If you don't worry about what you look like, you'll get on with your work. For the most part, the young Japanese women at All Nippon Airways and other companies don't seem to mind that American women are in a "special category." But why don't employers make more of an effort to nurture Japanese women instead of hiring "foreign" women? According to the Japanese government, Japan has a skilled-labor shortage. Mr. Takanobu Teramoto, labor attache at the Japanese Embassy in Washington, D.C., explains that in 1989 the immigration restrictions were loosened so companies could hire foreign workers with specialist training - a creative but incomplete solution to changing demographics and resistance to training indigenous women. Experts say that, eager to shed its reputation as xenophobic, overly traditional, and insular, the Japanese government is encouraging corporations to improve their international image. This means hiring American and European workers. Most American women head back to the States within five years. Says Patty Salvagio of Foreign Executive Women, a networking group in Tokyo, "If you are here indefinitely, you won't have a lot of chance to move up. Japanese companies are still very traditional." For American women in Tokyo, "socializing" means going out for prearranged work-related group dinners and that's it. All spouses and significant others are left at home. "I had good social relationships with men and women from work," recalled Pamela Fields by phone. A former manager of International Corporate Services at Matsuya, one of Japan's largest department stores, she says, "I did have an affair with a Japanese man, but not from the company. Not only did I not get brought home, but the family actually had me investigated at one point." Those that stay on and marry Japanese men are the exception.