The Washington Capitals will be trying to make up for a lost past when they finally enter Stanley Cup playoff competition tonight in the opener of a best-of-five series against the New York Islanders at Nassau Coliseum (8:05 p.m., WDCA-TV-20, WTOP-1500).

It has taken nine seasons, the previous three of which finished in frustrating fashion, for Washington to qualify for postseason play. In their ninth campaign, the Islanders were drinking from the Stanley Cup--for the second time.

Now the Islanders are trying to capture the Cup a fourth year in a row, a feat accomplished only by the Montreal Canadiens from 1956 to 1960 and 1976 to 1979.

There could hardly be a more difficult assignment for the Capitals' first venture into hockey's deep waters. Besides the Islanders' tradition of excellence, they have humiliated Washington in the teams' last three meetings by scores of 8-3, 6-2 and 7-1.

Although they cannot deny the frigid facts of the matter, the Capitals are quick to point out that past performance in hockey is no guarantee of future success--or disaster. In 1980, for example, the Soviets wiped out the U.S. Olympic team, 10-3, two weeks before America's heroes won the gold medal at Lake Placid.

"Last year with Montreal, we won, 6-2, in Quebec in the fourth game and then went home and they knocked us out in overtime," team captain Rod Langway recalled yesterday before the Capitals donned new "Workaholic" T-shirts and prepared for a late afternoon flight to New York. "We were discouraged about the way we played against the Islanders last week, but that was last week."

"As bad as it's been the last few times, and particularly that 7-1 game last week, I think that's better than if the guys had played their butts off and lost 4-3 or 3-2," said Washington Coach Bryan Murray.

"We know there's not that much difference between us. We know they're a class team, but they aren't six goals better. We have a lot of room for improvement. We're going to play much better, I'm sure of that."

Although a lot of folks think the Capitals have accomplished a great deal this season merely by reaching the playoffs, the players do not agree.

"Winning the Stanley Cup is what the year is all about," said Doug Jarvis, who earned four Stanley Cup rings in Montreal and also experienced the stigma of first-round ouster the past two seasons. "We've got to win this series and go on to win the cup to make it a successful year. Anything short of that and it's an unsuccessful year. Winning the cup is what people remember."

"Unless you make the final four, you're lost in the shuffle," said Brian Engblom, another ex-Canadien who owns three cup rings. "The funny thing is, the further you go, the easier it gets. Everybody wants to move ahead and win at least a series or two, and that puts extra pressure on you. Once in the finals, everybody settles down and plays hockey, and the best team wins."

Murray considers the first 10 minutes of tonight's game to be critical. During most of the Islanders' victories over Washington, they have come out flying, scored some quick goals and virtually wrapped up the decision early.

"A lot of our guys seem to have problems in the first 10 minutes," Murray said. "They're a little tight or something at the start. It's going to be a particular problem tomorrow, because so many will be in their first playoff competition. The first 10 or 12 minutes are going to be a great influence on how we react, maybe for the whole series.

"The Islanders have to win those first two at home. The whole thing comes down to whether we can get one out of there, steal it or whatever, and move the home-ice advantage from them to us."

After the games on Long Island tonight and Thursday, both of which will be televised, the teams will move to Capital Centre for contests Saturday afternoon and Sunday night. The fifth game is scheduled for Nassau Coliseum Tuesday.