Start talking about who gets hit hardest under the American tax system, and you're headed for a certain argument. Everyone has his own idea of how the tax system is biased. And there are enough seemingly-conflictly statistics around so no one has to come out a liar.

A liberals, the big shortcoming in the tax system is that so many of the tax breaks go to the rich. To conservatives, the problem is that wealthy persons have to shoulder so much of the total tax burden. To some, the system is far too progressive. To others, it's not

The problem has come to the fore in recent weeks in the face of new studies from the Treasury Department and the congressional Joint Committee on Taxation that highligh specific aspects of the overall U.S. tax structure - and not always in ways that pleases everyone.

The surveys, reported separately in a series of articles in The Washington Post over the past two-and-a-half months, show these major findings:

Just as conservatives have been contending, higher-income taxpayers do indeed pay the bulk of all federal income taxes.

Statistics show the richest one-fourth of American taxpayers - those with incomes of $17,000 a year or more - took home half the reportable income in 1977 and paid 70 percent of all federal personal income taxes - as astonishinsg figure by any meausre.

By contrast, those in the poorest one-fourth-wage-earners making less than $5,000 a year - received less than 5 percent of reportable income in the nation and paid a minuscule 0.1 percent or less of the total federal income tax tab.

But, just as the liberals have been complaining, the richer taxpayers receive the lion's share of 'the special tax breaks - deductions, credit s and other kinds of writeoffs - that the income tax system has to offer.

Of an estimated $84 billion in special tax breaks last year, almost half went to the 5 percent of all taxpayers with incomes of $30,000 a year or more. By contrast, taxpayers earnings $10,000 or less got only 12 percent of all tax breaks - much of its steaming from Social Security retirement credits.

Despite the "progressive" structure of the federal income tax system - which it taxes the rich more heavily than the poor - the federal income tax isn't the great income leveler it's commonly thought to be.

While the income tax does hit wealthier persons proportionally harder than less-affluent ones, on average it has relatively little effect in redistributing income in this country, as some liberals would like to see.

For example, the richest one-fourth of American taxpayers took home 55.5 percent of the reportable income last year before federal incomes taxes were taken out. But after taxes, they still had 53.2 percent. The income tax had relatively little impact.

When federal Social Security taxes and state and local taxes of all kinds are included, the nation's overall tax system becomes "proportional" - that is, it taxes both richer and poorer taxpayers at about the same rate - 31 to 33 percent of their total income.

It's only when government "transfer payments" are taken into account - welfare and Social Security benefits and other major programs - that the system actually shifts significant amounts of income from rich to poor.

How can all these findings be correct? And what does it all mean?

Well, confusing as some of these conclusions may seem at first blush, tax experts say they aren't contradictory at all.

It shouldn't surprise anyone, for example, that the wealthier segment of society pays the bulk of the total federalincome tax tab. After all, that's the way it's supposed to work: The system taxes only those persons with income. Those who have larger incomes theoretically are taxed more heavily than the rest. The only surprise is the extent to which richer taxpayers bear the burden.

Nor is it any great wonder that the more affluent taxpayers claim the bulk of the special tax breaks. The bigger your income, the more every deduction is worth in dollars. Any many big tax breaks are skewed toward investments, which are made mostly by persons with money.

The reason the federal income income tax isn't much of an income redistributor is that except for the few extremely rich and extremely poor, the tax rates paid by most income groups aren't that dramatically different from one another.

The high tax rates listed for those in the upper income brackets rarely are the ones actually paid. In the first place, wealthier taxpayers can reduce their taxable income by dedeuctions. And the highest rates apply only to the upper portions of a taxpayers's income.

Finally, the impact of Social Securities taxes and state and local taxes - to convert the overall tax system from progressive one to a proportional one in which all taxpayers pay roughly the same percentage - isn't much of a surprise, either.

Both the Social Security tax and state and local taxes are extremely "regressive" - hitting the poor harder than the rich. As a result, their effect is to offset - or at least nullify - the mild progressivity of the federal income tax. And so it goes on.

As might be expected, each segment of the taxpaying public takes umbrage at the notion that the other fellow may be getting hit harder, and ballyhoos those statics that conform most to what it wants to hear. Conservatives emphasize that richer taxpayers pay most of the taxes. Liberals stress that the burden isn't skewed enough.

The fact is, however, that each of these findings is a valid one, and illustrates some point about the structure of our tax system.

What these studies shoW, collectively, is that the tax system that's evolved is a complex patchwork of compromises that doesn't really please any segment of the population completely.

On the other hand, neither does the system go overboard in fulfilling the special goals of any one group or segment.Its main impact seems simply as a conduit for collecting taxes - shortcomings and all.

That may not be a very satisfying assessment of a tax system we've come to pride as a social model for the Western world - the "fairest," as campaigning contend, in all the free world.

But it may help explain why everyone from President Carter to liberals and conservatives in Congress has been having such a tough time drumming up much sentiment for radical change.