LEAD: In a crudely partitioned loft above a store in Brooklyn, six women listen as their instructor, Linda Ellman, tries to tell them what to say in a job interview when the conversation turns to previous work experience.

In a crudely partitioned loft above a store in Brooklyn, six women listen as their instructor, Linda Ellman, tries to tell them what to say in a job interview when the conversation turns to previous work experience.

The six have rarely held jobs. They are single mothers, in their 30′s and 40′s, supporting themselves and their children mostly on welfare. Only one has a high school diploma, although four others have passed high school equivalency tests.

They represent what officials call the city’s worst labor problem – unemployed single mothers.

At a time when the city is creating more jobs than ever, two-thirds of the single mothers of working age in New York City -more than 210,000 women in all – live in poverty. Most are black and Hispanic women and they struggle to survive on welfare or in jobs that do not pay enough to pull them above the Federal poverty level. Need for Decent Jobs

Counting only the mothers and their children under the age of 18, these households represent nearly 10 percent of the city’s 7.2 million people, according to data compiled by the Bureau of Labor Statistics and others. No other group is growing as fast -in New York, and in some other major cities as well – and that has made households headed by single, impoverished mothers increasingly the center of an effort to bring the mothers into the work force.

”If you want their kids to do better, then the women themselves have to have decent jobs,” said Frank Levy, an economist and demographer at the University of Maryland.

Toward this goal, the recently issued report of the city’s Commission on the Year 2000 recommends that single mothers facing years on welfare be required to work, return to high school or enroll in a job training program. Congress is considering legislation, including a bill sponsored by Senator Daniel Patrick Moynihan, that would finance more job training, work placement and education for welfare recipients.

In the loft in Brooklyn, all the women are determined to get jobs that pay more than the minimum wage. Their determination has brought them to a state-funded program, Women in Self Help, known as WISH, which operates as a combination employment agency and support center, teaching them how to hunt for work and cheering them on after each discouraging job interview.

”When you have not worked,” Ms. Ellman, the instructor, said, ”you have to explain to the job interviewer what you personally have to offer. Often they go for the person who can fit in.” ‘Could Keep Their Check’

And often the mothers do not fit in. The Brooklyn center expects to place only 70 women, out of more than 150, in jobs this year, said Carol Marsh, the director. The dozen or so other organizations operating similar programs for single mothers in New York City do not do much better.

But forcing single mothers into jobs can miss the complexity of their skills and experience. Sonia Baez, who is 40 years old, has raised three children on welfare, with the youngest, a 15-year-old, still at home and the two oldest, in their 20′s, now in the Army.

Ms. Baez, who separated from her husband while she was a teen-ager, had learned typing at a business school years ago and she typed students’ papers at home to supplement welfare payments that totaled $300 a month last year, not including food stamps, under the Aid to Dependent Children program.

Last September, she was told that the welfare payments would stop unless she raked leaves and did other chores for 53 hours a month at a park near her Brooklyn apartment.

”That blew my mind,” Ms. Baez said. ”I told them I had been looking for secretarial work and they could keep their check.” She said her caseworker reconsidered and allowed her to receive benefits while she continued to look for a job. The Gamble of Employment

After unsuccessfully answering classified ads, Ms. Baez turned to WISH for help. Their she learned to sell her bilingual skills, and after 20 job interviews arranged by WISH, she found secretarial work paying $12,000 a year at the Catholic Guardian Society in Brooklyn. The job includes health insurance, which Ms. Baez and her son lost when her income disqualified her from welfare.

The risk of losing health insurance for themselves and their children makes many single mothers hesitant to work. ”It is hard to find a job that covers as much as Medicaid,” said Alair A. Townsend, New York’s deputy mayor for finance and economic development.

Addressing the problem, the bills before Congress would continue Medicaid coverage for nine months after a welfare recipient is disqualified because of income.

Some New Yorkers going off welfare get the nine-month extension now, but coverage can be cut off in four months under current rules. That makes going back to work a gamble, which some of the mothers take.

There are other deterrents as well, chief among them the low-level of pay that many unskilled women receive as file clerks, restaurant workers, day care counselors, mail sorters and the like. Very Little Incentive

A mother of two earning the minimum wage of $3.35 an hour in a 40-hour-a-week job would be paid less than the maximum welfare payment of $652 a month for a family of three. To rise above the Federal poverty level, that same mother would have to make at least $4 an hour.

William J. Grinker, Commissioner of the Human Resources Administration, which administers the city’s welfare program, acknowledges the problem. ”For many people, there is a psychological boost from getting a job,” he said, ”but there isn’t much benefit from moving off welfare when the job doesn’t pay much beyond the minimum wage.”

In fact, it can result in a considerable setback when child care costs are figured in, along with the eventual loss or curtailment of Medicaid coverage. Many experts argue that a single mother of two must earn between $10,000 and $15,000 a year before she can justify giving up welfare for a job.

Judith O’Dell, who has been on welfare for five years, since her second daughter was born, hopes to earn $10,000 or more a year. After her first daughter was born 17 years ago and she dropped out of high school, Ms. O’Dell had avoided welfare, pursuing a singing career while her mother took care of the child. Problems of Training

Ms. O’Dell was successful, landing various jobs here and in Canada, including a stint as a background singer in a Broadway production of ”Elvis, the Legend Lives.” Between jobs, Ms. O’Dell’s mother, a retired hospital worker, kept daughter and granddaughter going.

”I still sing,” Ms. O’Dell said, ”but now it is for the church.” To make a living, the 35-year-old woman wants a secretarial job and she has begun training in typing, shorthand and business math at the Wildcat Service Corporation in lower Manhattan. It is the biggest of the nonprofit organizations trying to bring the unskilled into the work force.

The problem of earning enough raises the issue of how to train single mothers for the sophisticated jobs that New York’s economy, dominated by banking and financial services, seems to be creating in abundance.

City officials and Mr. Moynihan argue that the public sector should underwrite the cost. An alternative is for businesses to give unskilled women on-the-job training.

But with a few exceptions, such as Manufacturers Hanover Bank and Chemical Bank, the city’s private employers are not recruiting among the unskilled.

Employers find ways to get around additional hiring, said Walter Stafford, a senior researcher at the Community Service Society, one of the city’s oldest social welfare organizations. ”What I have found is that employers can make adjustments, improve productivity and avoid hiring these people,” he said.