Yang Sung U, a South Korean poet in his early 30s, wrote a long poem entitled "Diary of a Slave" which depicts his countrymen as impoverished and oppressed by the government.

It could not have been published in his country but Yang managed to get it printed in a Japanese magazine last year. He was arrested, tried and sentenced to three years in prison for violating President Park Chung Hee's proclamation prohibiting criticism of the government.

Paik Nak Chong's crime was publishing a book of essays about life in China, written by people who have traveled or lived there - John Kenneth Galbraith, Harison Salisbury, Edgar Snow and others. Paik was charged with violating a law that forbids the writing or publication of material praising communism. He is on trial now and faces a possible seven-year sentence.

The authorities were more lenient with Park Yang Ho, whose offense was a long alegorical short story called "Crazy Bird." Modeled on George Orwell's "Animal Farm," it portrayed South Koreans as frightened chickens guarded by dogs who reward their submissiveness with handouts of food. Park was arrested but have been effectively banned because according to a Ministry of Justice official, he had displayed repentance.

As these cases suggest, the life of South Korean dissident writers is not an easy one. The government views critical writing with the same distaste it displays toward street demonstrations and often metes our stern punishment for both. At least half a dozen writers and publishers have been arrested in recent months for works considered hostile to the government.

South Korea has no formal prior censorship law, but a number of books have been effectively banned because the Ministry of Culture and Information refused to permit their distribution and sale within South Korea. The ministry requires that any published book be submitted for approval before distribution. It can prevent its circulation entirely or require changes in editorial content.

Most publishers avoid the risk by declining to publish anything the government might not approve, one dissident asserts. A single offensive passage can mean that a book must be rewritten, reprinted and rebound.

Ministry of Justice officials agreed recently to discuss the charges placed against several authors but declined to talk about the government policy that punishes critical writings.

In the past, other officials have said that dissent - written or spoken - must be tightly restricted. The reason, they said, is that their criticism of the government as a sign of weakness. This prolongs the division of Korea, they maintain, and might even encourage the North Koreans to wage war against the South.

Few of South Korea's writers have international reputations and the obstacles they face are hardly known outside their circle of friends here. Since political criticism is strictly prohibited outside of the National Assembly by laws and proclamations dating from the early 1970s, many writers have turned to poetry and allegorical fiction as vehicles for their protests. Even those veiled criticisms have provoked arrests and trials, however.

South Korea's most famous poet, Kim Chi Ha, is serving a life term for having allegedly manipulated a student demonstration against the government in 1974. He was released in 1975, but then indicted again and sent back to prison for two of his works. One was a newspaper article suggesting that eight persons hanged as revolutionaries in 1974 had been framed by the government.

The other was unpublished play which the government found among his papers. Huh Hyong Koo, director of the Justice Ministry's prosecution bureau, said last week the play was considered a criminal offense because it claimed that "class struggle is bound to end in victory for the proletariat." Kim, who is back in prison serving the original life sentence, is appealing the conviction and the seven-year sentence he received for his latest writings.

Pai Nak Chong, whose small company published the book essays on China, and the book's editor., Lee Yong Hee, are now on trial. The book, called "Dialogue With 800 Million People," is describe by their friends as a harmless collection of writings by noncommunists.Huh, the Justice Ministry official, said editor Lee had inserted "some of his own observations" which "supported the inevitability and the legitimacy of a communist revolution."

Lee is also accused of espousing Mao Tse-tung's philosophy in another book which he wrote called "Idolatry and Reason." Lee is a former journalist and college professor who was forced to abandon both careers because of his political views and writings.

One dissident interviewed last week said he found some signs that the government was easing slightly its restraints on writers. He cited the case of two who published antigovernment works of a young poet and were arrested for violating a presidential proclamation. They were released after a month in prison.

The case of Park Yang Ho, who wrote the satirical story "Crazy Bird," is also mentioned as a sign that the authorities are lessening the punishment for critical writers. Those who have read the story describe it as a clever and imaginative allegory and say it enjoyed considerable underground popularity before the government determined that it was potentially dangerous.

The "crazy bird" of the title is a chicken who tries to persuade the other chickens that they can fly to freedom and escape their watchdog guards; a thinly veiled suggestion that South Koreans should attempt to assert their civil liberties. In the end, the bird" fails and is killed by one of the dogs.

The charges against author Park were dropped after he had been imprisoned for a month. Huh, the Justice Ministry official, said the charges were dropped because "it was his first offense and to the prosecutors he had to look of being repentant."