The United States does not plan to give up its new MX and Trident II long-range missiles in the Strategic Arms Reduction Talks (START) that are to open with the Soviet Union next week in Geneva, a senior American official said yesterday.

Rather, the official said, the United States is offering Moscow as an incentive in the arms talks an opportunity to limit how many of these new and highly accurate weapons will be deployed eventually.

"It is a matter of degree," the official said. He said the United States needs the land-based MX, the submarine-based Trident II and a new bomber, and therefore "we are going ahead" with them. "I don't envisage being in a position to say we are prepared to give up one of those systems," he said.

"What we are offering," the official said, is a reduction in the overall number of warheads on U.S. missiles, which also would mean deployment of fewer missiles than might otherwise be the case. The administration proposal calls for both the Soviets and this country to cut back the number of warheads now on their missiles by about one third, to no more than 5,000 for each nation.

The American official, who asked that he not be identified, also provided reporters a glimpse of what goes on behind the scenes at arms negotiations and why they tend to move so slowly.

He said, for example, that it probably would take the U.S. delegation a month just to lay out its opening position in the formal twice-a-week plenary sessions, even though the main outlines of the Reagan administration proposal have been made public.

He also said that the real clues to Moscow's thinking are likely to be gained not at the formal sessions but by "listening between the lines" at private meetings after the formal sessions.

"That's where the skill of this thing" really occurs, the official said, referring to the one-on-one back-room meetings between individual American civilian and military delegates and their Soviet counterparts. "That's where the 'what if' and the 'what do you think,' the 'I don't like this and you don't like that' kind of thing takes place that is much of the real negotiating."

What is said at the formal sessions is binding on both delegations. It becomes official policy. Thus, the United States team may take five to eight formal meetings to lay out its official proposal in bite-sized, step-by-step fashion. Translation into Russian can double each hour-long statement, after which the Soviets get the floor to ask questions or respond.

If experience is any guide, the official said, the formal sessions then will break up into individual meetings between each delegate and his counterpart. There the crucial exploratory discussions will take place. Although all of this is reported to Washington and presumably to Moscow, it is not binding.

It is in these private sessions, the official said, where skill is required to find points of flexibility in the other side's position. Sometimes it is revealing just to find out which views are sent back to Moscow and to which power center in the Soviet bureaucracy.

The official said he did not know if the Soviets also would put forward a detailed proposal at the opening round of talks. But he said that if experience is a guide the Soviets are not likely to go much beyond the general principles already outlined by Soviet President Leonid I. Brezhnev, and will be more interested in listening to the specific U.S. ideas.