A year after Prime Minister Pieter Botha began telling whites that South Africa must make changes in order to avert a black revolution, his government is at an impasse in discussions on constitutional reforms with its nonwhite majority and is facing increasing internal instability.

The Botha administration's shattering of raised expectations has reduced the government's credibility with its nonwhite population, increased racial polarization and dimmed hopes among white and black moderates that a peaceful solution to this country's problems can be found.

The hopes Botha raised a year ago by his bold sermons on the need for change "have all gone down the river, because people aren't seeing any changes," said Arnestine Majibe, a black maid, reflecting a widely held sentiment.

Signals of disillusionment also come from white quarters not usually given to public criticism of the government's policies. For example, the South African Jewish Board of Deputies warned recently that the country "may well erupt into violence and bloodshed," if changes are not introduced quickly.

There are many signs to support the gloomy assessment. More people are in political detention than a year ago. The South African Institute of Race Relations records more than 400 detentions since March 14, although this number fluctuates as people are released and others detained.

The Army has begun guarding certain strategic installations after the country's worst case of industrial sabotage when guerrilla mines set off huge fires at the government's fuel manufacturing plants. This followed the seizure of white hostages in a Pretoria bank and increased assaults on police stations and police officers.

Worker and student unrest continues in a sporadic pattern in various parts of the country, holding out the potential of violent clashes with police and repetitions of the rioting that occurred last month in Colored (mixed race) ghettos around Cape Town.

New restrictions were imposed on the press during the last parliamentary session. Official authorization now is required before newspapers can publish information on security arrests, on sabortage of strategic facilities and on police activities to quell unrest.

A commission was appointed to determine whether current press legislation meets the "demands of the times" a step many observers see as a harbinger of more restrictions.

Meanwhile the most significant "reform" of Botha's program, the president's council, intended as a forum for discussing a new constitution in South Africa, has been rejected by Colored, Indian and black leaders, even those who usually work within government-created institutions, as well as by the official white opposition party.

These groups have shunned it because it does not include blacks and because it is designed to work out a constitutional order based on ethnic divisions, a premise Botha and several other Cabinet ministers have underlined.

Supported only by Botha's ruling National Party, the council idea has proven useless for getting nonwhite support for a new political order.

To make matters worse, Botha tried to breathe life into it with threats, saying in a recent television interview that the only alternative to the council is confrontation for which "South Africa would pay the price."

How did Botha go from a honeymoon period in which blacks were praising him for his "courageous" remarks on the need for change, to the uneasy stalemate of today?

The explanation lies primarily in the fact that Botha's policies do not aim at the equal citizenship and political power-sharing nonwhites are demanding.

Instead, Botha has proposed a more authoritarian government, concentrating greater powers in his own hands in order to partition South Africa along ethnic lines, and a confederation ruled by a political elite of whites and their selected nonwhite allies.

This ethnic-based goal was stressed repeatedly in the last parliamentary session by ministers who spoke of "sovereign nationalities" and "vertical differentiation" as "non-negotiables." To nonwhites, there are synonyms for apartheid.

Furthermore, a government commission stated that it would take "a decade at least" for this evolutionary process to be completed.

There have been changes under Botha. Blacks allowed to live in urban areas now can move from one city to another without asking permission from the government. Black and multiracial unions are officially recognized. The wage gap between races in the civil service is rapidly being narrowed. More money is being allocated to improve conditions in black areas. nRestrictions on black businessmen in urban areas have been lifted.

But all these changes are on the periphery of where the nonwhites' attention is focused: equal citizenship and a negotiated, not dictated, process leading to nonracial power-sharing.

There has also been disappointment in some areas with the retreat Botha has made from a showdown with his right wing. Under pressure from the right wing, Botha has dropped his outspoken calls for whites to accept a new order and he refused to remove any discriminatory legislation in the last session of Parliament.

Right-wing opposition is also blamed for Botha's decision not to include blacks on his president's council and not to rebuke or fire a Cabinet minister who told Parliament that blacks have "slower thought processes than whites."