Before the New York Islanders line up to be fitted for their fourth Stanley Cup rings, they'll have to survive an obstacle course of teams determined to end their three-year domination of the playoffs.

The Islanders and their first-round opponents, the Washington Capitals, won't begin play until tomorrow night, but two of the NHL's eight playoff series will open tonight.

Boston will open at home against Quebec and the New York Rangers will open in Philadelphia. The other six series will start Wednesday. Islanders-Capitals

Thanks to the much-publicized deal with Montreal that brought in Rod Langway and Brian Engblom, and to rookie Scott Stevens' steady performance and to improved goaltending, the Capitals improved defensively. They finally will reward their faithful followers with a playoff appearance.

Offensively, the Capitals will probably count heavily on a line centered by Dennis Maruk, with Bengt Gustafsson and Mike Gartner on the wings. "Against the Islanders, you need one line, one key line," Washington Coach Bryan Murray said. "We expect things from the (Bobby) Carpenter line, but I'm hoping that one (the Maruk line) will be the key."

Still, after losing their last three meetings with the Islanders, the Capitals cannot anticipate an easy time on Long Island. Even if New York did finish behind Philadelphia in the division, and didn't get 100 points for the first time since winning the cup, the Islanders still led the league defensively.

Even if Bryan Trottier hasn't had a spectacular season, he still gets the puck to Mike Bossy, who scored 60 goals this season. To get anywhere, the Capitals will have to win at least one of the games in Nassau Coliseum, where they last won in December. Quebec-Boston

Last season, the Nordiques rudely eliminated Montreal in the early round, then defeated the Bruins. Finally, they lost to the Islanders. This season, Quebec finished fourth in the Adams Division, but remained a scoring threat. Michel Goulet had 57 goals, and the Stastny brothers--Peter, Anton and Marian--and Dan Bouchard's goaltending will hardly give the Bruins an easy first round.

Boston goaltender Pete Peeters, who was obtained from the Flyers last year, is given most of the credit for the Bruins' performance this year. He led the league with eight shutouts and had the lowest goals-against average--2.36. Defensively, the Bruins may have some problems. Defenseman Randy Hillier tore ligaments in his right knee in Saturday's game in Montreal, and a week ago, defenseman Mike Milbury broke his right kneecap.

Boston was still the only team to win 50 games this season, and is still mentioned often as the team most likely to succeed the Islanders as champions. New York Rangers-Philadelphia

The Flyers' rookie goaltenders, Pelle Lindbergh and Bob Froese, had been hot all year until a recent cool spell sent Philadelphia's chances of finishing atop the league tumbling. Coach Bob McCammon was at a loss to explain his club's late-season struggles, but wonders if the Flyers had "reached all their goals a bit early."

The Rangers have struggled, too, this season, although they won their last two meetings with the Flyers. Herb Brooks' team also has questionable goaltending, and injuries have been troublesome the last eight games. Eight players are being recalled from the Tulsa farm to provide enough bodies for the first round. Winnipeg-Edmonton

Last year, the Edmonton Oilers were eliminated from the playoffs early. The Los Angeles Kings, who had finished 48 points behind the Oilers in the regular season, may have taught the Oilers a lesson. Wayne Gretzky said his team has learned enough not to repeat last year's mistakes.

The Jets wound up 32 points lower than Edmonton, scoring 311 goals to Edmonton's league-leading 424. Among all of Gretzky's records, he hasn't yet had his name engraved on the Stanley Cup, but if his postseason production pace comes near his regular season play (71 goals, 125 assists, 196 points) he may not have to wait too long. Chicago-St. Louis

The Black Hawks finished the season with the third-best record in the league, thanks in part to Denis Savard, their third-year center, and Steve Larmer, the leading candidate for rookie of the year. Larmer scored 43 goals, two in Sunday's playoff preview victory over the Blues. Buffalo-Montreal

When Scotty Bowman coached the Montreal Canadiens, they regularly paraded around the Forum with the Stanley Cup. But Bowman is now coach and general manager of the Sabres, and although Montreal hasn't paraded much since he left, Buffalo hasn't won any cups, either.

Montreal's Guy Lafleur has scored only 27 goals in each of the last three seasons, far off his earlier productivity. The Canadiens have gotten used to beginning summer after only one round of the playoffs, and nothing would please Bowman more than seeing that trend continue. %&Calgary-Vancouver

Last year Vancouver went to the finals against the Islanders; this season, Roger Neilson's team will begin against the Flames. Lanny MacDonald has played inspired hockey for Calgary all season and says he plans to do more of the same in the postseason. The Canucks have struggled lately, and hopes of another trip to the finals are slim for this up-and-down club. Minnesota-Toronto

The North Stars changed coaches nearly halfway through the season but stayed within a game or two of the Black Hawks all year, trying to finish ahead of them for a Norris Division title. But Minnesota fell short, despite the increased output of rookie Brian Bellows.

The North Stars should have little trouble ousting the Maple Leafs, who lack staying power. Mike Palmateer is back in the nets, and he did take them to a seventh-game semifinal with the Islanders--but that was back in 1978.