The Metro board unanimously adopted yesterday a 10-cent increase in the rush-hour bus fare for riders boarding in D.C. and a comprehensive fare schedule for the expanding subway and bus system.

The action by the board eliminated the last remaining political obstacle to the opening on July 1 of a 12-mile train line from RFK Stadium to National Airport. The new fare schedule will take effect the same day.

The seven-year-old basic bus boarding charge of 40 cents in D.C. goes to 50 cents during rush hours. It will remain at 40 cents the rest of the day.

Suburban bus riders, many of whom will be forced from their buses to the new subway in July, will find that for the most part the total round trip busrail fare is about the same as they are paying for bus-only trips today. It will be marginally more in some cases, marginally less in others.

Subway riders will find that they have a basic boarding charges of 40 cents at all times. That 40 cents buys them three miles. During rush hour, each additional mile will cost the rider 7.5 cents. The rest of the day, each additional mile will cost 3.75 cents.

Because of the complexisties of the fare-collecting system, people who must take first a bus, then a train, then a bus to reach their destinations will be severely penalized under the fare structure adopted yesterday. That problem still must be resolved, Metro board members agreed.

Computerized fare collecting equipment will be able to determine where the rider entered the subway, where he left, and how much he owed. The computer will deduct the fare owed from the rider's "Farecard," a magnetically coded, wallet-sized device. The rider can buy Farecards in any subway station.

Yesterday was the deadline for the Metro board to decide on fares. Metro officials will need the time between now and July 1 to program the computers being installed at all subway stations.

Metro general manager Theodore Lutz promised that riders who use the existing 5-mile subway downtown will get a chance to practice on the new equipment before the formal opening date of the expanded system.

The most dramatic change in regional transit patterns in history will accompany the July opening. Virginia commuters will be encouraged to board Metro trains at the new Rosslyn, Pentagon and National Airport stations.

Commuters from Southeast Washington and Prince George's County will be encouraged to transfer at the new Potomac Avenue Metro station.

Many bus lines that presently run from the outlying areas into downtown will be curtailed at those subway points. Many other buses will be rerouted so their passengers have ready access to the subway.

To eliminate some of the confusion, the Metro board has decided to continue running its present bus schedule to July 17 - thus giving transit users a 16-day period of adjustment.

The fare structure adopted yesterday will also apply Nov. 1 when another 5 miles of elevated Metrorail are scheduled to open between the exiting Rhode Island Avenue station and Silver Spring Similar plans for curtailing and rerouting feeder-bus service are being made.

However, Metro board members are still deciding what to do about an apparent inequity in the fare structure that would exist for Maryland residents using the Takoma Station, just inside the District line.

Also adopted by the Metro board yesterday was a formula to share the costs of operation the subway that exceed the amount of revenue raised. In the first year of the new system, that deficit is expected to be $22 million.

The formula covers three years of operations and changes the percentages each jurisdiction must pay based on several factors, such as population and the number of stations in the jurisdiction.

Most of the operating deficit is funded by local governments, who have to depend on the real estate property tax for most of their revenue. There has been a growing revolt among area politicians to devoting real estate taxes to transit operations.

That was never more evident than yesterday. Prince George's County Executive Winfield M. Kelly and Council Chairman William B. Amonett sent a letter to Metro Chairman Francis White telling him that "the county subsidy payments under this formula will not be paid from the real property tax. Therefore, if no alternative sources of revenue are available . . . Prince George's County will not guarantee payments." White, coincidentally, is also a Prince George's County councilman.

The political problems will appear slight, however, when compared with the perplexities that the commuter will face in understanding the new fare structure. Philosophically, it presents at least three different views.

Area politicians have adopted, for the moment, the suggestion that the subway fare should be regional in concept. It costs the same no matter who you are, where you live or where you get on.

But the subway ties into a bus system that has a totally "Balkanized" fare structure, to use the word of Lutz. Virginia jurisdictions are committed by policy to recovering two-thirds of their bus operating costs from the fare box. D.C. has been collecting about half of its costs, and subsidizing the rest. The Maryland suburbs have fallen somewhere in between. The new fare structure continues to reflec those divisions when bus and subway rides are tied together in one transit trip.

Further complicated the system is the fact that the subway computers cannot accept transfers, only Farecards. However, transfers acceptable to bus drivers will be issued by machines at the subway stations.

A typical rider will board the bus, pay the fare to the closest train station, enter the train station (and his fare in the computer) and be at his destination. Notice, he has paid two full fares.

Coming home, he gets half of it back. He enters the subway with Farecard, gets a transfer to the bus, leaves the train station (and his fare in the computer) and boards the bus with his transfer. Only one full fare. Thus, the round trip cost will be about the same after July 1 at it is today, although one-way cost may be higher, the other way lower.

The Maryland commuter gets a free ride home anywhere in Maryland with a bus transfer from the train station. The Virginia commuter must pay zone fares on the bus, but not the base fare, reflecting the conflicting philosophies of subsidization.