On Monday morning Metro will change or eliminate 210 Washington-area bus routes and drop thousands of sleepy, confused commuters into the subway whether they want to take it or not.

It is the first phase of Metro's revolutionary plan to turn its massive bus operation into a feeder service that complements, rather than competes with, the train.

The Metro subway's new Blue Line from National Airport through Rosslyn, Foggy Bottom, downtown Washington, New Southwest and Capitol Hill to the RFK Stadium area has made all this possible.

Trains have been running on the Blue Line for one month now. Despite continuing mechanical difficulties that make every rush hour ride an adventure, an average of about 100,000 riders a day is using the subway's two lines. Before the Blue Line, it was 30,000a day.

Because of those mechanical difficulties. Metro delayed Monday's bus changeover for two weeks. It had originally been scheduled to happen on July 18.

"We've got to do it sometime." Metro general manager Theodore Lutz told the Metro board Thursday when he was asked if the subway's reliability was adequate to take the crush of bus commuters. Delays of 20 to 30 minutes have been common in the first weeks of Blue Line operation.

For many commuters, the total cost of a round trip to work using the bus and the train will be more than a bus only round trip by 30 or 40 cents. For many others the cost will be the same or even 10 cents less.

Metro does not know how many persons will be affected by the change Monday. It had estimated that some 50,000 bus riders a day from the 210 routes would wind up on the train. Metro now thinks that maybe 20,000 people have already made the switch without the coercion of the changes in bus routes.

Whatever the figure, there is certain to be an enormous amount of uncertainly not onlyon Monday but for several days as public transit users figure out what has happened to them.

Those who have not practiced will get their first introduction to Farecard, Metro's electronic automated fare-collecting system that requires a rider to buy a ticket he must use at both ends of his subway ride. During the past two weeks more Farecard equipment has been installed at keystations, but it will be November before all Farecard machines are in place.

The Metrobus fare structure, already mysterious because of the differing subsidies Washington area jurisdictions give their resident bus riders. becomes even more complex when commuters figure in the cost of a subway ride.

There are no transfers for riders going from the bus to the train. They will have to pay two fares. But riders going the other direction from the train to the bus - are able to transfer. They must, however, pick up their transfer from a vending machine at the Metro station where they board the train or the bus driver will not honor it.

Train riders who transfer to buses for destinations in the District of Maryland will get a free bus ride after leaving the train, no matter how far they go. Riders who transfer from the train to Virginia-bound buses will have to pay bus zone charges.

The following examples all use higher rush-hour fares, which Metro charges between 6 and 9:30 a.m. and between 3 and 6:30 p.m. weekdays:

An Oxon Hill resident who boards the bus there, transfers to the train at Stadium-Armory and travels to Metro Center will pay a total round-trip costs $2

A District of Columbia resident who takes the bus to Potomac Avenue and transfers to the train for Metro Center will pay $1.40 for a round trip, as opposed to the $1 it costs now. Before July 1, when the D.C. bus fare for rush hour increased, that round trip cost only 80 cents.

A McLean resident who rides the bus to Rosslyn for a train trip to Federal Triangle will pay a total fare of $2.10 by the time he or she returns home in the evening. A bus-only trip would cost $2.20.

Anybody who wishes will able to stay with bus, although transfers may be required for what are now through routes.

While the trains are running between 6 a.m. and 8 p.m. weekdays, Metro is distically reducing parallel bus service. The reason is to cut bus costs and to increase rail revenue. Metro figures the total savings in the fiscal year that began July 1 will $4 million.

The new fare structure, however, falls most heavily on the area's poorest residents - those in Southeast Washington and Southern Prince George's County.

Metro is still studying various longterm bus plansthat would ease the fare burden. The Metro board is alsoseeking a way to get around a quirk in the fare system that would make a bus-rail-bus round trip from Anacostia to Friendship Heights cost $2, instead of the 80-cents it cost June 80 by bus alone.

About 2,000 signatures appeared on a petition that Southeast residents gave Metro last week, They asked that the present bus system remain intact until rail reaches them.

D.C. City Council Chairman Sterling Tucker, a Metro board member, said that "I have heard from some constituents about this . . . I think they're being good about it but a 30-cent minimum increase for a ride which may take longer somehow does not seem right."

What is happening to all 210 bus routes that are changing Monday does not lend itself to summary. Both Washington newspapers have run complete lists of bus route changes and Metro has been handing out information on route changes and new time-tables to bus patrons.

Generally, here is the plan:

Bus routes that cross Key Bridge will be terminated at the Rosslyn Metro station.

Bus routes that cross Key Roosevelt Bridge will be terminated at the Frederal Triangle Metro station.

Bus route that cross the Sousa Bridge (Pennsylvania Avenue SE) will be terminated at the Potomac Avenue Metro Station.

Bus route that usethe Rhode Island Avenue of New York Avenue corridors will be terminated at the Rhode Island Avenue Metro Station.

Other bus route will be terminated at Federal Center SW, Farragut West, Union Station and the Pentagon Metro stations.

A total of 33 routes, most of them one-trip-only rush-hour designations, will be eliminated.

Most of the buses will revert to their traditional through-routings when Metro is not running. Although most Key Bridge service will be terminated at Rosslyn, a new route, M6, will run regulary between Rosslyn and Union Station.

Most routes serving Alexandria, south Arlington and southern Fairfax County across the 14th Street and Memorial Bridges willnot change until Sept. 6.