WORLD PRICES of copper, sugar, coffee and cotton have all dropped severely over the past couple of years, and the trend is continuing. It's nice for the industrial countries that import raw commodities. But it's very hard on those countries whose fortunes depend totally on the international market for one or two products.

Just as unemployment rates are the measure of social distress in the industrial countries in times of economic decline, the commodity prices are the measure of the distress in the poorer countries, closer to the equator, that live by exporting raw materials. For the Third World, there's an important difference between the last recession and the current one.

The last time the world's economy sagged, in 1974-75, most of the Third World governments managed to keep business expanding at home by borrowing heavily. The banks had a lot of money to lend because the OPEC countries, after the first great oil price increase, had more cash coming in than they could immediately spend. But this time, the Third World won't be able to expand its borrowing. The accumulation of debt is already large, and the banks are getting anxious. There is no longer an enormous OPEC surplus to be borrowed, for the OPEC countries have rapidly become accustomed to their new affluence. Spending money is an art easily learned, and some of them are even running deficits.

With no new borrowing, the Third World can adjust to its declining export income only by retrenchment at home. The effects will not be limited to standards of living and development in the Third World alone. It's going to make a difference here in the United States as well.

Because the Third World borrowed in 1974-75, it was able to keep buying a heavy flow of goods from this country. That, in turn, helped revive employment and business profits here. In 1982-83, that isn't likely to happen. As countries get fewer dollars for their copper, sugar, cotton and so forth, they will have fewer dollars to spend here. It's another reason to doubt that the world is going to see, in the year ahead, the kind of strong and steady recovery that began to take shape seven years ago.