SOUTH Africans tomorrow begin talking hard about the shape and governance of their country's future. For the first time in 340 years, blacks and whites will together attempt to decide constitutional principles and how to achieve a nonracial, harmonious commonwealth.After more than 40 bitter years of apartheid, reconciling the pent-up demands of 30 million Africans, Coloreds, and Asians with the fears of 6 million whites will hardly be easy or smooth. President Frederik de Klerk and his ruling white National Party want to move forward, but without undermining either the national economy or prevailing "standards" of white life. Africans, led by Nelson Mandela and the African National Congress (ANC), seek the franchise denied to them for more than 50 years. Together with the ability to vote, they want at least to share power, if not to wield it exclusively in proportion to their overwhelming numbers. Immediately, Africans seek to transform the meetings Dec. 20-21 into the launching pad for a constituent assembly. The preferred or ideal African plan would have Mr. De Klerk concede power next year to a neutral, transitional interim government. That white-black body would hold the country together while all South Africans went to the polls and elected an assembly to write a new constitution (Namibia did precisely that in 1989). But whites and the National Party do not intend and, from their point of view, cannot afford to relinquish power prematurely. Clearly, whites want inasmuch as possible to control the process of devolution, to hinder unfettered black hegemony, and to manipulate sessions like this one from their vantage point as the government. Based on these radically contrary bargaining positions, an impasse could easily ensue. However, although neither side wishes to budge, neither can afford deadlock. Prosperity in South Africa depends upon a resolution of the political crisis. The country cannot take full advantage of the ending of economic sanctions, the cessation of sports boycotts, and the removal of its pariah status unless these talks succeed and negotiations continue. As strong as the government may be militarily, apartheid has collapsed. Both sides realize that South Africa cannot forge ahead economically or culturally without progress politically. As weak and disorganized as the ANC may appear, legitimacy and numbers are on its side. And neither camp wants to risk delaying a settlement into 1994, when another whites-only election might then have to be scheduled. Conservative white political parties are boycotting tomorrow's meeting. They have little political future if the National Party and black groups successfully achieve solid understandings this weekend and devise constitutional and interim proposals early in the new year. Aside from the Conservative Party, 21 of the 30 invited delegations from mainstream political movements as well as from the homeland governments are expected to participate in these all-party talks. Each of the 21 will have three delegates. The ANC is only the largest and most important black political group at the talks. Originally the government hoped that both the liberal white Democratic Party and Chief Mangosuthu Buthelezi's Inkatha Freedom Party would oppose the ANC's extreme demands. Leaks by the government have also disturbed the unity of the "Patriotic Front" composed of the ANC and the much smaller and more radical Pan Africanist Congress, which refused to attend. But the Democratic Party has recently moved closer to the ANC, and Inkatha's credibility as an independent entity has been compromised severely by new revelations that the government funded and trained Inkatha "hit squads." Those hit squads have attacked ANC adherents and fomented urban violence for five years. The Democratic Party now favors the calling of a constituent conference elected on the basis of proportional representation and a universal franchise. So does the ANC, but probably not the homeland representatives and Inkatha. Nor does the government. Nevertheless, either this weekend or in continued discussions in the new year the negotiators will have to find a vehicle by which a new constitution can be written. Then - sometime next year - the real battle will occur over whether undiluted majority rule will prevail or, as the government naturally prefers, majority rule will be limited by a minority veto. In addition to constitutional questions, the all-party talks must consider how to reincorporate the "independent" homelands into South Africa proper and the appropriate kind of a national bill of rights (the government-appointed South African Law Commission recently surprised whites by proposing a strong, entrenched bill of individual, not group, rights). The delegates will doubtless debate how to move forward, not the decisions of the real endgame. Then they will reconvene in the new year and spend weeks, if not months, mutually fixing what will surely become a new South Africa.