The Washington Capitals will not finish unbeaten in 1980. They will not win their first game in Nassau Coliseum this season, either. The New York Islanders took care of both eventualities tonight by whipping the Capitals, 5-2.

In many respects it was a replay of the Islanders' 2-1 triumph here three weeks ago. The Capitals had an edge in play, outshooting New York in every period and posting a final margin of 38-31, but Islander goalie Chico Resch proved virtually impossible to beat.

It was 4-0 before Rolf Edberg and Mark Lofthouse found the scoring formula in the third period and that was too much of a deficit to overcome, even if the Capitals did bounce back from 4-0 to beat the Rangers at Capital Center four weeks ago.

"Miracles only happen once and they tend to happen in our building," said Washington Coach Gary Green. "We can't get four goals behind here and expect to win."

After two victories and two ties in the new year, the Capitals came here with high hopes of making their 12th visit the first winning one. Even after Dave Lewis sent New York into a 1-0 first-period lead, there was no letup.

Then two plays took the heart out of the Capitals, First, after goalie Rollie Boutin had made some brilliant saves during an Islander power play, a bad bounce boosted New York's lead to 2-0.

Stefan Persson lined a shot off the rear boards and the puck caromed out over the goal line, to be tipped in by Bryan Trottier for his 24th goal.

At the other end, Bob Sirois wheeled around New York defender Bob Lorimer and fired at Resch, who blocked it. The puck was still loose, but referee Ron Harris thought Resch was on it and blew his whistle, a moment before Guy Charron swept it into the net.

Goals 61 seconds apart by Wayne Morrick and Mike Bossy, No. 25, doubled the load before the second period ended and the Capitals were out of it.

Green spent most of the first period trying to match lines with Islander Coach Al Arbour and even stationed men on the ice just outside the bench to change on the fly as soon as the puck was dropped. If the maneuvering looked ineffectual, Green felt it at least slowed the Islanders, who had come out buzzing.

"I was stubborn tonight and by being stubborn we slowed their game down to a snail's pace," Green said. "Then our game plan was to come out for the second period, when they were expecting a snail's pace, and take it to them. But it never happened."

The Capitals, who took their fists out of storage while thrashing Vancouver, 7-4. Friday night, were involved in more of the same tonight.

Rich Green, who rarely resorts to fisticuffs, demolished the Islanders' Duane Sutter with half a dozen punches. Green then needed repairs to his right hand, cut on the visor Sutter uses to protect a battered right eye.

"I hit it once and I hit his head a few times," Green said. "I'm not one to look for fights because I figure I'm more valuable on the ice than in the penalty box. But a few guys try to take advantage of my good nature and I have to show them. I asked him kindly not to bother me. then I had to show him."

Ryan Walter exchanged wrestling holds with former Capital Gord Land, as Lane reacted to each Walter check by swinging fists and stick. Walter admitted he made no effort to avoid the man who walked out on the Capitals earlier this season.

"He gave me a couple of cheap shots the last time we played, so I guess you could say I was the instigator this time," Walter said. "He gave me a couple of cheap shots. I said I'd get him and I did. We're even. He intimidates, tries to get people off their game. That's his job.I did mine."