Americans have learned plenty about the environment in the past year, since the 20th anniversary of Earth Day, April 22, 1990. Some developments have been encouraging. Some have not.

Here's the most discouraging news: The more Americans think they know about how they should protect the environment, the less sure they are about how they can really do it.As the 21st Earth Day approaches, it's apparent that new knowledge has created even tougher questions for environmentalists to answer in the months ahead.

- Cloth diapers are supposedly the best way to swaddle babies. Disposable diapers are out. Yet some scientists now contend that the manufacturing and washing of cloth diapers create more pollution than the use of disposbles.disposables. Who's right?

- Paper cups have replaced foam cups as the product of choice for some fast-food restaurants. Yet new evidence indicates foam cups may damage the environment less than paper cups. A Canadian chemistry professor, writing in "Science" magazine in Febrary, noted that manufacturing a paper cup "consumes 12 times as much steam, 36 times as much electricity and twice as much cooling water as a polystyrene foam cup." Which is better, really?

- Companies are marketing biodegradable products such as garbage sacks that supposedly break down in the landfill as being better for the environment than old-fashioned plastic sacks. Yet recent tests reveal that biodegradable products generally just break up into smaller pieces when subjected to the heat and microbes of a landfill environment. "People have jumped to the conclusion that biodegradable plastics are good, but the truth is, they don't biodegrade," said William Jewell, a Cornell professor who wrote a report earlier this year on the subject. Are biodegradables an unnecessary fad?

- Recycling programs have sprung up around the nation. More paper, plastic, aluminum and glass are being reused. Yet critics say these programs are extremely expensive for the amount of trash removed from the waste stream. Until solid markets for recyclables are established, opponents contend, recycling is just one more non-noteworthy achievement for eco-radicals who want to feel good about the pains they are taking to protect the Earth.

Sorry, but it's necessary for environmental crusaders to be realistic about the troubles they face. They need to determine what the most important problems are and then convince other Americans and environmental skeptics that action is required to reduce pollution.

Enough doom and gloom. It's time to realize that the last year has provided positive news for environmentalists and for their efforts to stem pollution.

The state of Missouri, for instance, approved new guidelines aimed at creating regional compacts that will strive to recycle up to 40 percent of the waste stream by the year 1998. Yard waste, for instance, has been banned from landfills by next January. Compost heaps are going to spring up around the state.

Nationally people have been passing programs and tax increases aimed at preserving the environment. Advocates earlier this month compiled information to indicate that environmentalists succeeded in approving good programs or rejecting bad ones 59 percent of the time in 1990. Environmentalists are not out of touch with American voters, went the rallying cry.

Although the complex Big Green legislation failed in California, five industry-sponsored propositions in that state lost as well. Other programs were approved. They included the Arizona Heritage Fund to protect the state's natural and cultural resources, the Minnesota Environment and Natural Resources Trust Fund and the Nevada Parks and Wildlife Bond Act.

Other intangible benefits have occurred in the last year. Generally, there is more awareness that what people do, what they consume, can have an effect on the environment. Those effects can be reduced.

Critics of the environmental movement say much of the reaction to modern-day pollution is hysterical, that costly programs spring up when common sense ought to take over. Sometimes opponents have good points. They should be acknowledged when accurate.

But it's also true that the air, water and land need to be protected. More and better information needs to be gathered on how to do it. The easy answers hoped for 20 or so years ago have proved elusive. It's obvious that a responsible environmental movement will be needed in the United States for a long, long time.