PRESIDENT REAGAN believes that he saved you from a tax increase this summer when he demolished the Republican senators' budget plan. But did he? Look again. He has imposed a different tax, and it is now in place.

The Reagan administration is now borrowing more than $200 billion a year to finance its budget deficit. Somebody is going to carry that burden. Who? President Reagan has spared you the kind of tax that Congress enacts -- the kind that is predictable, limited and distributed with some concern for fairness. By ruling out that kind of a tax this year, he has only ensured that the Debt Repayment Tax of 1985 will be of another kind.

One possibility is a surge of high inflation. Many people in this country already assume that the solution to the accumulating burdens of federal debt will be simply to inflate the currency again, as in the 1970s. In that case, those debts will be paid off by savers and investors as they watch the value of their money shrink. Inflation is a hidden tax but a real one -- and it falls blindly, with no consideration of ability to pay. The Reagan economic program was supposed to encourage saving and investment. Instead, it is making saving and investment much more risky.

But perhaps that surge of inflation will never happen. Fear of it is now sufficiently widespread that at the first hint of it interest rates will soar. Lenders and investors lost gigantic amounts of money in the inflation of the 1970s, and they will send interest shooting upward as they attempt to protect themselves from a repetition. If that happens, instead of inflation there will be a long and severe recession. In that case the tax will be paid by the working people who lose their jobs and the companies that go bankrupt. The Reagan economic program was supposed to generate a steady rise in employment and expansion for business.

In either case, Americans are also going to have to carry the foreign debts that this country is now running up because of its unbalanced domestic economy. The Commerce Department announced yesterday that the trade deficit in June was once again extraordinarily high. The longer Mr. Reagan lets these debts continue to rise unchecked, the heavier the repayment taxes will ultimately become.

The next tax has already been enacted, by a mechanism that -- unlike Congress -- is not open to argument and appeal. The only question is the precise form of the tax, a detail that will eventually become clearer. Mr. Reagan has prevented Congress from trying to devise a tax of its own. He has guaranteed the alternative -- a tax that is blind, unfair, unforgiving and hostile to every element of the stable prosperity that Mr. Reagan originally hoped to build.