The promises by Japan yesterday to liberalize trade with the United States mark only the beginning of a long negotiating process that almost assures constant friction between the two nations in the months ahead.

The current round of talks essentially began last Jan. 2 when President Reagan and Prime Minister Yasuhiro Nakasone met in Los Angeles in an effort to open Japan's markets to U.S. products in four areas in which the administration said it believed competitive American products could sell well if high tariffs or other restrictions were lifted by the Japanese.

The four areas cited by the Reagan administration were telecommunications, forest products, sophisticated electronics, and pharmaceuticals and medical equipment. The president has made it clear, however, that the list is not exclusive. Other trade issues still facing the two nations are such longstanding problems as U.S. access to the Japanese markets for tobacco, beef and citrus products and Japan's access to the U.S. market for its steel and auto products.

The sense of urgency for resolving the trade problems stems from the record $123.3 billion U.S. trade deficit. Nearly one-third of the deficit -- $36.8 billion -- is the result of Japanese exports to the United States.

Of the four areas of trade concern outlined by Reagan and Nakasone in January, negotiators have concentrated most of their initial efforts on telecommunications. Negotiators were faced with an April 1 deadline because that was the date the Japanese had set for developing new regulations for its newly denationalized telecommunications industry.

* Telecommunications: While most knowledgeable Americans, both in and out of government, agree that U.S. negotiators achieved unprecedented access to Japan's decision-making process, there is a wide divergence of views on whether this will mean added sales -- the prime objective of the talk. Much depends on how the powerful Japanese bureaucrats implement the new regulations.

There have been some breakthroughs. According to a U.S. Embassy translation, Nakasone yesterday for the first time listed "harm to the network" as the goal for simplifying the standards for telecommunications equipment to be sold in Japan. That is the only standard used by the Federal Communications Commission in the United States to govern what equipment can be sold in this country. It means that any equipment used should not cause technical breakdowns to the system.

But some skeptical U.S. trade negotiators see two "choke points" remaining for U.S. sales. One is the requirement that companies have to notify Japan's Ministry of Post and Telecommunications before they can do business. Some see this notification as a device to provide information about U.S. companies' intentions to their Japanese competitors or as a bureaucratic device to hamper foreign businesses. The other potential problem is the standards adopted by the Japanese, which are subject to continuing negotiations.

There are still other telecommunications trade issues that were not subject to the April 1 deadline but must still be negotiated. A major one involves the Japanese government's refusal to buy U.S. satellites to give its own infant space industry time to mature. With the denationalization of Nippon Telegraph and Telephone (NTT), however, the new private companies are now allowed to buy U.S. satellites, and Nakasone pledged yesterday that they will. It remains unclear, however, whether the Japanese government will place either formal or informal limits on the number of satellites its private companies can buy and on what frequencies.

Then there is perennial issue of the four-year-old agreement for NTT, then a government monopoly, to increase its purchases of U.S. supplies. U.S. Trade Representative William E. Brock said yesterday that U.S. and Japanese officials will begin consultations Monday on "the unsatisfactory level of NTT purchases."

Japan's radio laws also must still be tackled. These govern sales of mobile telephones, an area in which Japanese products have flooded the American market. But no mobile telephone that meets common international standards can be sold in Japan.

* Sophisticated electronics: A major U.S. concern is the protection of U.S. computer software. U.S. companies hold a competitive advantage, but the government fears Japanese companies will move ahead of the United States by pirating software and eliminating nontariff barriers to Japanese companies buying the most sophisticated American-made computers.

Some negotiating progress has been made in this area. Under more than a year of intense U.S. pressure, Japan agreed to provide copyright protection for U.S. software programs. This is seen by the United States as a defensive measure against possible piracy, not a plus for opening U.S. markets.

The same is true for a semiconductor chip protection measure, which is expected to pass the Japanese Diet (legislature) soon.

On another issue, the Ministry of International Trade and Industry (MITI) has proposed the mutual elimination of tariffs on electronics products, marking a possible plus in an area in which the United States asked for an end to tariffs on electronic goods such as computer parts and test equipment.

MITI has not responded to a U.S. request to speed up Japan's patent process so U.S. companies can win design protection in the rapidly developing high-technology areas.

Five other U.S. requests still have not been addressed in the talks: American participation in the development of industrial standards; acceptance of U.S. test data; participation for American companies in research and development projects sponsored by the Japanese government with access to the patent information they develop; streamlined customs procedures for electronic goods, and the revisions of foreign exchange controls that make it hard for Japanese companies to buy new equipment from the United States.

* Forest products: This is probably the most difficult area to unravel because it involves a politically potent, highly protected Japanese industry that would likely be driven out of business if less expensive American products were allowed in Japan.

The talks broke down in February after Japan refused to discuss eliminating tariffs, but Nakasone yesterday proposed a five-year adjustment program with lower tariffs starting in 1990. This is seen here as a way to get negotiations going again, not as a breakthrough.

This is, furthermore, an issue that has angered influential U.S. senators such as Robert Packwood (R-Ore.), chairman of the Senate Finance Committee, and Max Baucus (D-Mont.), both from states with large timber interests.

* Pharmaceuticals and medical equipment: Nakasone agreed yesterday to allow foreign test data to be used as criteria for allowing medical equipment and some diagnostic chemicals "because they are immune to ethnic differences." But pharmaceuticals still require expensive and time-consuming test data developed in Japan.

Six U.S. requests were laid on the table last month with Japan's Ministry of Health and Welfare. With the exception of the test data issue, the other requests still are up in the air.

Japan is boasting about solving one irritant to its trade relations with the United States -- a series of restrictions its tobacco monopoly placed on U.S. companies. Many of these were lifted with the abolishment of the 80-year-old Japan Tobacco and Salt Corp., but its replacement, a joint stock company called Japan Tobacco Inc., still manages the domestic industry. It is unclear whether that will lead to more U.S. sales.