WHAT WITH ONE THING and another, we've been finding it increasingly difficult lately to get a firm grasp on the state of this country's cherished, longstanding alliance with South Korea. Let's see if, just once, we can get it straight:

On the one hand, the Carter administration wants to engage in a progressive withdrawal of American ground forces from South Korea. The Joint Chiefs have agreed to it - but reluctantly, and only if the withdrawal is gradual and accompanied by a substantial increase in U.S. military aid. The South Koreans are not at all reconciled to it, and neither are a lot of members of the U.S. congress.

On the other hand, there is the matter of Tongsun Park, the South Korean rice-trader and influence purchaser who is by way of being a fugitive from American justice. A U.S. grand jury has indicted him for a variety of offenses connected with his alleged campaign contributions to American political figures, most of them past or present members of the Congress, and identified him as an agent of the South Korean government. The U.S. Justice Department wants the South Korean government to deliver Mr. Park to this country to stand trial, and also to be available as a potential witness in other court proceedings against some former and perhaps even sitting members of Congress who may have illegally benefited from Mr. Park's keen interest in the American political system. The special counsel of the House ethics committee, Mr. Leon Jaworski, is also extremely anxious to have Mr. Park's help in his investigation of the South Korea's non-cooperation can "only strain" its relations with this country, and President Carter is even said to have taken a personal hand in the efforts to effect Mr. Park's return.

But the government in Seoul says it has no official control over Mr. Park and that when it recently tried to appeal to his better instincts, he simply wouldn't listen. So there doesn't seem to be any immediate prospect of Mr. Park's coming back to his old haunts in Washington. And this, in turn, has caused some members of Congress to talk about witholding the first installment ($114 million) of that extra military aid for South Korea that is supposed to compensate for the removal of U.S. troops. Fully 181 congressmen yesterday voted, in symbolic form, to make such a cut, and this is not likely to be the end of it.

At about this point the plot begins to thicken - when you consider the probable consequences of using the military-aid money as a bludgeon in this matter. The first consequence of withholding the Military aid is almost beyong question: The Joint Chiefs would withdraw their support for the troop withdrawals. The second consequence is about as predictable: Without the extra military-aid money and the crucial support of the U.S. military, the Carter administration could not realistically hope to be able to follow through with its withdrawal policy.

Some bludgeon. And some result: Tongsun Park would remain silent in South Korea, which would not exactly dismay certain members of Congress; American troops would stay on in South Korea, which would not displease an even larger number of congressmen with strong misgivings about the wisdom of the withdrawal plan; the Congress, which is justifiably nervous about its reputation for high standards of official conduct and a willingness to investigate its own improprieties, would have given fine show of its deep concern for justice and ethics and all that sort of thing. And the South Koreans, by obstructing the course of judicial proceedings and a congressional investigation in this country, would also have a fair chance - if the military aid were ultimately to be withheld in retaliation - of obstructing President Carter's Korean troop-withdrawal plans as well.

Small wonder that South Korea's President Park chung Hee was last seen by a couple of visiting House members beaming happily and making had jokes about Tongsun Park's "human rights." He will go on smiling until it is made clear to him that what is at stake here is not a cut in American military aid or a domestic American scandal but rather publicsupport for the American commitment to South Korea.