The number of American workers who can expect to receive pensions from their employers when they retire is rising dramatically, and the overall financial status of the nation's elderly is also improving, according to recent studies of retirement benefits.

But the complexity and variety of pension plans and retirement programs is also increasing, requiring many of today's workers to make difficult and sometimes risky choices that will determine their resources 20 or 30 years into the future.

A combination of new laws, tax rules, inflation and recessionary pressures on business is leading many corporations -- large and small -- to revise their pension programs, often transferring the long-term risk from themselves to the workers. As a result, pension experts are urging workers to begin their retirement planning early, and to acquire a thorough understanding of the options they have and the resources they can expect to command when they reach retirement age.

"A happy and satisfying retirement requires an adjustment period that is greatly aided by thoughtful, effective planning in earlier working years," the Employee Benefit Research Institute (EBRI) said. "Ideally, one should begin planning for retirement between age 30 and 40," a time when many workers find their resources fully obligated by the costs of raising children and buying a house, and when retirement seems remote.

In 1940, only 4.1 million workers (14.6 percent of the private, nonfarm work force) were covered by pension plans. In 1980, 35.8 million (48.1 percent of the private work force) were covered. According to a new study conducted by ICF Inc. of Washington for the American Council on Life Insurance, about 85 percent of all married couples of retirement age and 66 percent of all married individuals can expect to receive some employer-paid retirement benefits by the year 2004.

Many of those benefits are insured, up to about $16,000 a year, by the federally chartered Pension Benefit Guaranty Corp., which protects pensions the way the Federal Deposit Insurance Corp. protects bank accounts.

Furthermore, according to EBRI, despite fears about the future of the Social Security program, "during the last 20 years, the elderly's financial status has improved substantially. Today those who are over age 65 receive income from more sources and have greater financial independence than previous generations. Expansion of public and private retirement income and benefit programs has increased the proportion of those receiving retirement benefits as well as the average benefit amount."

Dallas Salisbury, executive director of EBRI, said the overall resources available at retirement now include more than monthly cash income and that these resources should be considered in assessing retirement prospects. Widespread price discounts and tax breaks for the elderly, mortgage-free home ownership, increased personal assets, and supplemental post-retirement health insurance should all be included in calculating retirement resources, he said.

The lingering recession has induced many corporations, especially smaller ones, to terminate their pension plans, leaving their workers substantially dependent on Social Security and personal savings upon retirement.

But the most dramatic change in the retirement income structure taking place today, pension experts say, is a growing trend among corporations to shift from what is known as a defined-benefit pension program to a defined-contribution plan. This means the employer, instead of being committed to paying a fixed benefit to future retirees, such as $12 a month per year of service, is committed only to making contributions of an unspecified amount to a retirement fund each month..

This shift represents "one of the major trends in retirement programs today," according to Susan Koralik, a pension trends analyst at Hewitt Associates, a leading actuarial consultant.

She said the defined benefit system would not immediately replace the majority of the fixed-benefit plans now in place at large corporations, but probably would command an increasing share of new employer contributions, both in existing plans and in new ones. Several major corporations, including GAF Corp., A&P, Triangle Industries, and Harper & Row Inc., have already switched over or announced plans to switch over to defined-contribution programs.

According to Koralik, Salisbury and other experts, individual workers need to be aware that this is a fundamental shift in their retirement benefits, not just an exercise in corporate accounting. It can have advantages and disadvantages for both employer and worker, they said.

For the corporation, it may reduce costs and eliminates any problem of unfunded pension liability on the balance sheet because by definition the contributions to the fund are equal to the liability. But it may also discourage older workers from retiring because of the uncertainty about benefit levels.

For the worker, defined-contribution plans usually offer earlier vesting and increased portability, making them attractive for those who expect to change jobs often. Also, the employe doesn't have to worry about losing his pension money if his company goes bankrupt because the contribution made to his account is in his name and beyond the reach of corporate creditors. At the bankrupt Braniff International Airlines, for example, the pensions of most rank and file workers are protected by federal insurance. But senior executives have had cut off that portion of their pensions exceeding $16,000 a year.

But the employe cannot know, until retirement, how much his fund is worth, especially if it is invested in stocks and valued according to market. And if the employe withdraws the pension money when changing jobs, careful decisions must be made about where to reinvest it.

The changeover at GAF appears to be typical of the way this pension plan shift works. GAF is planning to terminate a defined-benefit plan covering about 2,500 workers, whose benefits were vested after 10 years of service. The workers will have the option of taking their accrued benefits in a lump sum, purchasing a retirement annuity or rolling the funds over into a new defined-contribution plan. Here again, the workers face new and difficult choices.

Under the new plan, in which all workers are immediately vested, GAF will make a profit-sharing contribution equivalent to 3 percent of a worker's salary and will contribute additional amounts based on the proportion of salary a worker chooses to put into the retirement plan -- an inducement to save that also requires the worker to make a choice.

The 1982 tax law sharply decreased the amount of pension money that corporations may earmark for senior executives. According to Koralik, this is the result of growing "egalitarian" sentiment in Congress, which wants to broaden the benefit structure of the private pension system substantially subsidized by the government.

Corporate contributions to retirement programs are tax deductible as a business expense. What Congress is now saying, Koralik said, is that "we will continue the subsidy, but there is a social objective to it, namely providing retirement benefits, and therefore there is a real thrust toward seeing that more people are covered, toward decreasing what is given to the higher paid and increasing what is given to the lower paid."

Retirement planning experts also urge workers to examine the life insurance coverage of their employer's retirement program. Some plans continue to provide life insurance coverage after retirement and some do not. Workers whose retirement plans do not provide this coverage face a substantial additional expense if they purchase it on their own.