PRESIDENT ANWAR SADAT is a hard man to keep up with. Last November he was making the big gesture for peace with Israel and seemed to be ready to take some big risks. By January he was scuttling direct negotiations between Israeli and Egyptian foreign ministers, after only one day's haggling over fine print. Subsequently, he had a fling at playing inside politics with Israeli critics of Prime Minister Menachem Begin's intransigence, dangling hints of this or that concession before Labor Party leader Shimon Peres and Mr. Begin's own defense minister, Ezer Weizman, in a series of private talks that understandably infuriated Mr. Begin. The week before last, he countenanced a resumption of exploratory foreign ministers' talks with Israel in a castle outside London, under American auspices. While there was not much give or take, the American mediators on hand apparently found a new willingness on both sides to forego nitpicking and to try, at least, to address the large, hard questions.

And then, last week, Mr. Sadat was personally assailing Mr. Begin as "the only obstacle" to peace, abruptly expelling an Israeli military mission from Egypt and letting it be known both loudly and privately that he saw no purpose in a follow-up foreign ministers' meeting in the absence of some substantial softening of Israel's position. All this he was doing, we might add, right at the moment when Israeli Foreign Minister Moshe Dayan was demonstrating, in a full-dress report to the Knesset, at least a little more Israeli flexibility on the critical question of the future of the Israeli-occuppied West Bank.

What is one to make of this? At first blush, one is tempted to believe that Mr. Sadat has persuaded himself he can have it all, free of risk or sacrifice; a rapprochement with the hard-line "rejectionists" in the Arab World, a nice, warm, fruitful relationship with the United States, more and bigger Israeli concessions under American pressure, a hero's role as the architect of Mideast peace - the works. But since that is manifestly impossible and Mr. Sadat is more than smart enough to know it, the explanation presumably lies elsewhere. Part of it has to do with the way the negotiating process works; Mr. Begin has done his share of blowing hot and cold. And part of it no doubt has to do with inside politics on the Arab side. Mr. Sadat's latest zig - or zag - makes a certain amount of sense in tactical terms at a time when the Arab "rejectionists" are taking the occasion of a Belgrade meeting of "non-aligned" nations to try to haul Egypt back to the hard line. The Iraqis, the Libyans, the Syrians and others are angling, apparently, to do in the current Mideast peace process by somehow returning it to the tender, not to say smothering, mercies of the United Nations. When the hard-line Arabs are invoking the sacred name of Arab unity, it is hard politically for Mr. Sadat to resist the temptation at least to sound like one of the boys.

But he can't be one of the hard-line boys and make peace with Israel - or continue, for that matter, to be best friends with the United States. And while one may presume that nobody knows that better than Mr. Sadat, one cannot presume that he had not miscalculated the limits of American permissiveness. President Carter has been leaning heavily and conspicuously on Israel to take a more conciliatory approach - not without justification, in our view, but also not without some promising results. It is conceivable, observing this, that the Eqyptian president may have come to overestimate what the United States can be expected to extract from Israel and to underestimate what can - or will - be reasonably demanded of him by Mr. Carter, if a settlement is to be reached.

The time may be about right, in other words, for the United States, in the interests of constructive mediation, to begin leaning a little harder on Mr. Sadat.