KOREANS APPEAR stunned by the turn taken by Roh Tae Woo over the weekend. He is the fellow former general and, everyone thought, the rubber stamp of President Chun Doo Hwan whose nomination by the ruling party to succeed Mr. Chun produced the wave of great demonstrations that began on June 10. But suddenly Mr. Roh separated himself from his ostensible boss and mentor, offering to meet all of the opposition's major demands and stating that he would step down as both party chairman and presidential nominee if Mr. Chun did not accept his proposals.

In principle, anyway, Mr. Roh has cleared the way for an expeditious approach to democracy in a fast-modernizing, healthy and prosperous Third World country that was being increasingly poorly served by the military autocracy the Chun government and its predecessor had imposed for the better part of two decades. The key issue had become the manner of presidential elections. The generals had used an indirect system to retain power, while the opposition, more confident of its popular appeal, favored direct elections -- the kind Korea enjoyed until 1971. That's what Mr. Roh has now agreed to. He also proposes not merely to end the house arrest of dissident Kim Dae Jung but to restore his political rights and to relax the political scene in other substantive ways.

Mr. Roh's course seems calculated to detach the broad democratic opposition from radical, hard-to-conciliate students. To the extent that it succeeds, it may sharpen anxieties in some military quarters. The generals, after all, though they have offended the political sensibilities of many South Koreans, enjoy their respect for holding communist North Korea at bay. As for the opposition, it will be on notice to show it can compose its own considerable differences and put forward a sensible program in the new conditions evidently taking shape now.

Just a few days ago the Reagan administration appeared to have a potential disaster on its hands. Now it's a potential success. The difference springs from luck, which can still go either way, but also from a conscious effort to keep pace with the broad democratic passions that the students unleashed. President Reagan has been saying there is a democratic current loose in the world, and Korea is marvelous evidence.