The Social Security Administration has decided to try to send Americans letters that they can actually understand.

For years, beneficiaries have complained that the letters they receive from the agency, notifying them of such important matters as whether their benefits have been reduced or terminated, are often undecipherable, creating widespread confusion.

Now Acting Commissioner Martha A. McSteen has launched a drive to simplify the letters. Some have been rewritten and others soon will be.

For instance, one old form letter announced that there had been a benefit "adjustment" because of a change in the law -- but never explained what the change was. A box might be checked off that said, "Benefit payments have been discontinued with the month shown in column 2 for the reason shown above," namely, the unspecified change in the law.

Another box that could be checked said, "Benefit amount reduced to zero. Please see enclosure pertaining to change in the law," and gave the effective date of the action as "5/84."

The new version flatly tells the recipient: "Your benefits have stopped . . . . You will no longer be able to receive Social Security student benefits for months after April 1985. The April benefit is the one received about May 3."

It then explains "Why You Can No Longer Be Paid . . . . In 1981, the Social Security law was changed to phase out benefits for college students. The 1984-85 school year is the last year that college students can receive benefits."

Aides to McSteen said the agency still has a long way to go to clear away years of encrusted bureaucratic gobbledygook and obscure computerese. Social Security sends out notices to 26 million people a year.

"We expect, if everything goes smoothly, to put new language into effect for all of them within two years," said agency spokesman James M. Brown.