IT HAS been said that the best way to learn is to teach. But to learn a second language, the best way may be to shop and cook in the foreign tongue. Students studying English at American University, for example, have found the Safeway and Giant an intensive course, and cookbooks a tough exam.

The first thing Soegoro Prawirosumarto learned when he came to Washington from Indonesia five months ago was that you can't bargain in the supermarket. The second thing he learned was to cook. At home he had never cooked before, but the price of eating out in Washington convinced him to buy two pots at the supermarket and to prepare his own meals on his dormitory's two-burner stove. By now he is considered the chef of his dorm, but he still has trouble with shopping, especially with Giant's practice of not marking prices on packages. Sometimes, he explained, he has limited money, and wants to be able to keep a running tally of his purchases so that he doesn't come up short at the checkout counter.

The food Prawirosumarto cooks is familiar to him; he generally seasons his curries with packets of curry seasoning mix he has sent to him from Indonesia. But his breakfast has become all-American since the night he slept at a friend's house and was served cold cereal the next morning. It was such an easy breakfast that cornflakes has become his morning staple.

Yolandes Esquer is from Mexico, so supermarkets are nothing new to her, and she doesn't even mind the shelf-pricing system. But she does mind that "the fish in supermarkets is not fresh." She now buys hers from fish markets. She also minds not having the wide variety of fruits she found in Mexico, although she is impressed by the cleanliness of the produce in Washington's supermarkets. And she sorely misses the variety of chilies she found in Mexico. "We use 10 different kinds," she explained. But here she has found only serrano and jalapeno among fresh chilies, and almost never in what she would consider good condition. The variety of packaged American products, however, bewilders her. "When I came here I didn't know the brands," she recalled, so she spent months trying different spaghettis and laundry detergents before she settled on those she liked. She also is disdainful of the emphasis on frozen foods in Washington's markets.

Sure, the fruits and vegetables are clean, agreed Yllineth Paraguan from Venezuela, but "they put wax" on them, she shuddered. Even so, she said, "Supermarkets here are very well organized, and they attract the consumers. They decorate the food." Some of the technicalities of American supermarkets still confuse her. She is not quite sure what dates on packages mean, especially since she once bought pork chops two days before their pull-date and found them spoiled. But she likes the idea that the dates are there.

This is the second visit Olga and Francisco Velazquez have made from Mexico, so this time they brought their own spices, canned sauces and chilies, just as Jacqueline Netto's family brought their own flour from Brazil. American tomato sauces, Olga complained, have a lot of sugar. So she makes her own with a blender.

What do they like about shopping in Washington's supermarkets? Olga likes ingredient and nutrition labeling. "You can discriminate which can to use to reduce calories or salt," she said. And she also likes the fact that she can read a magazine while waiting in the checkout line. Francisco, who does most of the cooking, has found the supermarkets surprisingly similar to one another. As he put it, "You can forget if you are in Giant or Safeway because both have everything in the same place." In fact, he added, "If you were almost sleeping you can find everything you want."

Supermarket organization and efficiency bothers Anna Toure' from France. She is used to having butchers available, and likes buying unpackaged foods so that she can select individual items herself and can buy just the quantity she wants. "I can't buy my slice of ham," she complained. "Everything is packaged. If I want to buy two slices of ham and not more, I can't." Several of the students agreed, adding that in supermarkets in their countries they can taste the cheeses before buying. Thus they prefer to shop at ethnic groceries and specialty markets here when they can, but most of the time they find supermarket shopping too convenient to resist.

Their greatest disappointments center on fresh fruits and fish. "American food is -- uh -- not bad," said Sharifa Mahmood, here four months from Dubai. She misses fresh mango and melon juice, and wishes she could find more spices. But that is minor compared to her reaction to the fish. "It is not fresh, the fish," she declared. "In my country you can buy it after one hour. It is always very fresh in my country." And in Dubai, she finds many more kinds of fish. But if she thinks little of the fish, she thinks even less of American fast-food restaurants. "McDonald's in England are much better than here," she insisted. "The meat here is all fat."

Margareta Tornell of Sweden chimed in with a similar response. She came here from Sweden last fall to face a more difficult culinary challenge than the other students. She moved into the household of this food editor, and immediately began sharing the family's shopping and cooking.

One thing Tornell loves in Washington is the supermarket hours. "I've been here seven months," she said. "They've only been closed twice." She also praises their abundance. "They have everything."

She, too, is bewildered by the choice; when she went to buy beans, she found a half-dozen different kinds. "You have so many different kinds of salt and sugar, so many different kinds of milk and cottage cheese. You have 2-percent and 4-percent milk," Tornell bemoaned. She had to learn the colors of the milk cartons in order to choose the right ones.

"If I ask American people, they are so helpful," said Tornell of her bewildering moments at the supermarket. "But if I ask the people who work there, they just send me to aisle 8B."

Although Tornell likes the meat here, she has had trouble figuring out which cuts are tender and which tough. The variety of vegetables, of cereals, of fruits astonished her. As she grew used to the variety, though, she began to find things that delighted her. She was thrilled to find Finnish crackers like those she knew at home, and smoked salmon and herring. And, "Oh, the yogurt!"

To Viveca Ekstrom, also from Sweden, the yogurt tasted like whipped cream with jelly. Too sweet. Both Swedish students find American foods too sweet -- and too greasy. "Everybody who comes here gains weight," said Tornell of her friends. "There is sugar in everything."

Cooking has been a constant test of Tornell's English. In reading the newspaper she might be able to gloss over a word or guess at its meaning. But in cooking, interpreting "loosely" as "tightly" or skimming over a "not" can be disastrous. Her most embarrassing error occurred whenshe made Swedish beer bread with root beer instead of beer. Nobody said anything about the peculiar sweet taste of the bread until Tornell brought up the subject herself.

While American recipes are a pleasure for Tornell, with their step-by-step instructions, Ekstrom has been frustrated with recipes that simply direct, "Make a white sauce." As she put it, "Then I say, 'How shall I make a white sauce?' "

Both of them have called their grandmothers in Sweden to ask for recipes.

For foreign students, American measuring cups take some getting used to. Even Americans find it easy to confuse ounces of weight with liquid ounces, which are volume measures. Add to that the complexity of translating deciliters to cups and grams to pounds, and it is enough to daunt a math student, much less an English student. Tornell taped a metric conversion chart to the refrigerator, and has labeled her measuring cups.

These foreign students are also accustomed to less-processed foods. "Here you find many foods 'semi-done' because life is so fast," noted Dionisio Borda from Paraguay. In Latin countries, he continued, you have a lot of time to prepare food.

While Tornell and Ekstrom also deplore the American penchant for prepared mixes, they are enchanted by American machines. Tornell had never seen a garbage disposal before, and said she likes it a lot. "One thing I think is great is the microwave," exclaimed Tornell, pronouncing it "meecrowave." As for the food processor, "Oh, I use that many times!" She loves looking at all the machines in stores.

Both Swedish students, since they are living with American families, are learning a great deal about American tastes. Ekstrom is impressed that in her adopted household the father cooks. "When the father is cooking it is so good," she marveled. "He makes his own pasta."

And Tornell learned about American Thanksgiving -- the hard way. She eagerly took over much of the pre-preparation for the family dinner. One of the recipes she used was for potatoes, which she multiplied to feed 15 people. She enlisted two small girls for the peeling, and the project lasted all afternoon. After all, she figured, for 15 people one would need 40 potatoes.

Those two small girls are ready for Army mess duty, and the family ate potatoes halfway to Christmas.