President Reagan might not learn as quickly as some of us would like, but he does learn.

He learned from Jimmy Carter's Iranian crisis that weakness (or a reluctance to use power) can tempt your enemies into rashness.

He learned from Israel's longtime policy of retaliation for acts of terrorism (even if the particular terrorists are dead and their sponsorship is in doubt) that there is some value in having it known that you will respond.

He learned from the own response to the terrorist bombings of the Rome and Vienna airports that tough talk without the means for backing it up is just another form of weakness.

And now, in the aftermath of last week's bombing of a TWA jetliner, he is showing signs now of learning the hardest lesson of all: that there are some aspects of international terrorism that you really can't do much about.

The evidence that that lesson is sinking in is this: in his first Saturday radio address after the April 2 assault on TWA Flight 840, the president made no mention of the bombing, in which four Americans died. And though his remarks, centered on strengthening the U.S. military, he did not mention the bombing of a German disco frequented by American GIs, though that bombing had taken place just one day earlier.

That doesn't mean that Reagan has decided that America is helpless against terrorism. (In fact, his aides revealed Saturday that top administration policymakers were meeting that very day to discuss plans for dealing with terrorism.) It means only that he has learned that publicly announced threats are useless -- and maybe harmful.

It's too early to know whether the president's discreet silence means that he has chosen sides in the administration debate over how best to deal with terrorism: whether to do nothing except when, as in the wake of the Achille Lauro hijacking, it can be done with precision and to specific purpose, or, as Secretary of State George Shultz has advocated, to retaliate for every terrorist outrage and even to strike preemptively.

It may be that he is absorbing the lesson that, at the extremes, both kinds of response -- passivity and reckless retaliation -- are more likely to increase terrorism than to stop it.

Nor is there much reason to suppose that strikes against countries known to harbor terrorists would be effective. Israel practically destroyed Lebanon in that attempt, but still is vulnerable, at home and abroad, to terrorist attacks. In addition, many of the countries that provide safe haven for terrorist groups do so not because they support terrorism but because they fear becoming targets of terrorism. Just last week, it was revealed that both France and Italy made secret deals, since abrogated, to give Libyan-sponsored terrorists freedom to travel through Europe in exchange for pledges that their own citizens would be spared from attack.

Terrorism, particularly when practiced by those willing to die in the effort, cannot be combatted by military means, no matter how reckless or precise.

The best approach may be the two- pronged one of working as hard as possible to make our citizens less vulnerable to attack (barricades, better airport security and all that) while doing what we can to resolve the grievances to which terrorism is a desperate response.

Even that won't cure the problem in some of its more irrational manifestations. Probably nothing will. But it might make the United States a less inviting target.

The president seems to have learned at least half of that lesson: that tough talk and tough action not only don't work but can be counterproductive. Is he an apt enough pupil to learn the second half: that the best way to secure peace may be to work directly for it?

Maybe -- if he can get over the notion that terrorists are nothing more than savages whose claims to international justice can be reasonably ignored.