With the presumed tacit approval of the Soviet Union, Hungary yesterday applied for membership in the International Monetary Fund, a necessary prelude to application for membership in the World Bank as well.

The application came as a surprise to Eastern European experts, who had not expected the Hungarian move for at least a year. There was speculation that Hungary's application, which almost certainly required a go-ahead from Moscow, might have been accelerated as a consequence of the current tense Polish situation.

So long as Hungary stays within the Soviet foreign policy orbit, Moscow is willing to let it or other Eastern European countries take steps that will enhance their economic well being, and thus remove the temptation to follow the Polish example, these experts said.

The application comes just after last week's 25th anniversary of the Hungarian uprising of 1956, which was crushed by Soviet troops and tanks.

Hungary, as a member of the IMF and ultimately the World Bank, not only would be eligible for direct assistance by the two Bretton Woods institutions, but also would enhance its borrowing ability from private sources substantially, especially in the Eurodollar market.

The application to the IMF, announced here and in Budapest, will be considered first by the IMF executive board. The procedure calls for the executive board to submit a report to the board of governors in the form of a "membership resolution." The report would recommend what Hungary's "quota," or deposit of its own currency in the fund, should be.

After adoption of the membership resolution, Hungary would need to take the legal steps required to enable it to sign the fund's articles of agreement and to fulfill the obligations of membership in the fund. Among such obligations is one that requires a nation ultimately to make its currency fully convertible to the currencies of other nations.

But for an open-ended transition period, convertibility is not required. Once accepted into the IMF, a nation is eligible for membership in the World Bank. There are now 143 member countries of the lending institutions.

Earlier this year, China joined the World Bank-IMF's growing list of Socialist countries, which now includes Yugoslavia, Romania, Laos, Cambodia and Vietnam. All of these countries are in the "transition" mode, which does not require currency convertibility. A total of 54 other countries initially in transition have graduated to a convertibility status.

In January 198l, Washington Post correspondent Kevin Klose reported that achieving currency convertibility for the Hungarian forint was regarded within Hungary as a major hurdle that might not be possible until 1985. Klose quoted Akos Balassa, head of the State Planning Office economics department, as saying that for convertibility "there must be an appropriate world climate. We hope we can do it, but it's not like we have to do it by 1985. At present, we are working hard to establish the preconditions."

Hungary in recent years increasingly has edged away from a Soviet-style state-run economy toward a mixed economy, adopting some enterprise-related mechanisms and substantial decentralization. Some small-scale private business operations have been allowed to go ahead, and the government has encouraged private concessionaires and shops. Just as significant, the government has tried to abandon a split currency system for the Hungarian forint. The tourist rate of exchange is now close to that for commercial transactions.

The major step toward achieving a mixed economy with an improved standard of living came in 1968 when, under the leadership of Janos Kadar, the Hungarians adopted what they called the New Economic Mechanism.

Since then, Hungary's two-way trade has moved increasingly toward Western countries. Compared to a few years ago when 70 percent of Hungary's imports and exports were with the Eastern bloc, the split is now roughly 50-50.