The Metro Board adopted yesterday a complex new fare schedule for the bus and subway system that will mean increases for the majority of suburban commuters, reduction for some others and little change in fares for most District of Columbia residents.

The new fares will take effect July 2. At the same time, Metro riders will be able to use for the first time a "Metro flash Pass" that gives them unlimited bus rides and a prescribed amount of subway serive for two weeks.

The board also gave final approval to subway service on Saturdays and the extension of weeknight subway operations from 8 to midnight, beginning Sept. 25. Subway service on Sundays is still under discussion.

The only category that does not change is the (D.C.) bus fare. It will remain at 50 cents during rush hours (6 a.m. to 9:30 a.m. to 6:30 p.m. weekdays, excluding holidays) and 40 cents the rest of te time.

Subway fare for midday, nighttime and weekends will be 50 cents, no matter how long the trip. During rush hour, however, subway fares will actually be cheaper than nonrush hour trips for many short trips. The 40-cent fare will remain for the first three miles. For longer trips, the rate will increase by 1 cent a mile from the present 7.5 cents. That means, for example, that the rush-hour fare from Silver Spring to Metro Center will increase from 75 to 80 cents.

All subway passengers who cross the Potomac River during rush hour will pay a minimum of 45 cents - even if their trip is only one stop long, from Rosslyn to FOggy Bottom.

Farecard, Metro's automated, computerized fare-collecting machinery, will continue to calculate and collect the fares from magnetically encoded tickets.

Metro will also continue its policy of permitting a transfer from subway to bus but forbidding the transfer from bus to subway. The new Metro Flash Pass will ease that transfer problem and reduce fares somewhat for people who use the system every day.

The new fare schedule probably falls most heavily on Virginians, particularly those in close-in Alexandria and Arlington County. AN Arlington resident making a round trip by bus and subway to the New Southwest every day will have a 20-cent fare increase.

A Vienna resident going the same place will have only a 10-cent increase and a Greenbriar resident will pay 20 cents less for a round trip.

That is part of the Virginia compromise that was worked out last week after Fairfax County's Board of Supervisors took note of the fact that the last big fare increase, in March 1977, drove thousands of riders away from Metro.

The reduction in fares for long-haul bus riders is true in both Maryland and Virginia because bus fare zones have been combined in both states as part of the new fare structure. Furthermore, zone-crossing charges were reduced for outlying Virginia zones, but increased for the close-in zones.

In Virginia, zones 3 and 4 were combined into Zone 3, and the lower Zone 3 charges apply to both. Maryland combined zones 2, 3, and 4 into a new Zone 2 and will charge the lower Zone 2 fares.

Maryland commuters are going to lose one fare break they have had for a year. It is presently possible for a Rockville resident to board the subway at Farragut North, ride one stop to Dupont Circle for 40 cents, then transer free to a bus that takes him all the way to Rockville.

Starting July 2, that rider have to pay the zone-crossing charge in addition to using his transfer. That means that the round-trip bus-rail fare for residents of Bethesda and Hyattsville, for example, will increase by as much as 30 cents a day.

Maryland riders on the N7 and N9 express buses from Montgomery Mall and Korvettes in Montgomery County to downtown Washington will also be assessed a 25-cent surcharge for express service.

The uneven application of fares across the metropolitan area and the difficulty of transferring from bus to subway and back again stems from the complexity of the Metro organization.

The subway fare must be regional in character because subway revenues and costs are shared regionally and allocated among the Metro partners. But bus costs and revenues, for the most part, are specifically assigned by route to one jurisdiction or the other.

Virginia, therefore, has higher fares because it is Northern Virginia policy to recover two-thirds of the operating costs of Metro from fares. The District of Columbia government believes lower fares and higher subsidies are in its best interests and those of its citizens; Maryland generally falls.

The three new Metro Falsh Passes will go on sale at most Metro ticket outlets beginning June 18. The passes can be bought at that time for two-week periods beginning July 2 and July 16. Each pass must be used within the given two-week period or any value remaining on the pass will be lost. The three pases are:

The $10 one-zone base pass, good for unlimited bus rides within one bus zone anywhere in the region and containing $5 worth of subway rides. It will be particularly useful for District of Columbia residents.

The $16 Maryland-D.C. pass, good for unlimited bus rides between any point in Maryland and the District of somewhere in between.

Columbia and containing $6.25 worth of subway rides.

The $16 two-zone Virginia pass, good for rush-hour two-zone bus trips within Virginia and containing $5 worth of subway rides.

All of the passes are, in effect, glorified Farecards, with distinctive coloring and markings.

The new Metro fare schedule is somewhat lower than the one the board proposed at recent public hearings across the region. The decrease in proposed fares will mean added deficits of about $900,000 in the fiscal year beginning July 1, Metro estimates.

That will make the combined bus and rail deficit about $99 million in Fiscal '79. About two-thirds of that is bus deficit; about 20 percent of the total deficit is paid by the federal government under its nationwide urban transit aid programs. The rest comes from area taxpayers.