The New York Islanders, who won the Stanley Cup a year ago, conquered New York tonight. Moving ahead to stay on their first shot, the Islanders crushed the Rangers, 5-2, to complete a four game sweep of their hated rivals and once again advance to the Stanley Cup final.

The Rangers did not yield easily; down 4-0, they halved the margin before Duane Sutter's third-period goal wrapped it up.

The rabid Ranger fans would not quit, either. Late in the game they chanted, "Let's go, North Stars," in honor of the Islanders' probable final-round opponent, Minnesota.

This was a night when very little went right for the Rangers, starting with the warmup, when goalie Steve Baker was struck on the side of the head by a Nick Fotiu shot. Baker played anyway, but he had to suffer another headache when the fans sarcastically cherered routine saves after the Islanders had taken that 4-0 lead.

Fotiu was assigned to the press box before he could commit further damage but his numerical replacement, defenseman John Hughes, was charged with two costly first-period penalities that were quickly transformed into goals by the potent Islander power play.

The Rangers were going with six defensemen because captain Barry Beck was hurting from a strained back and Coach Craig Patrick felt his defense was getting tired. "We played three series with five defensemen, some nights less because of suspensions and injuries, and it caught up with us."

But if Patrick's moves repeatedly turned sour, Islander Coach Al Arbour seemed blessed with genius. Forty seconds into the game, he pulled left wing Bob Bourne off the starting line and sent John Tonelli out to join Wayne Merrick and Bob Nystrom. Naturally, Tonelli scored, connecting on the game's first shot at 1:02 after Nystrom eluded a Beck checked and set him up.

With referee Wally Harris permitting a lot of questionable play to proceed without penalty, the Rangers briefly shoved aside their concern for the Islander power play and tried to use the assault tactics that proved so successful against Los Angeles and St. Louis. Harris, however, bounced Hughes twice within three minutes for charging and roughly Mike Bossy, and each time the Islander supershot got his revenge by scoring on the power play.

In the second minute of the second period, the Butch Goring made it 4-0 on a short-handed breakaway. If the Rangers were not dead, they were scarcely breathing and the 17,380 faithful were remarkably quiet.

Power-play scores by Ron Greschner and Beck revived the folks, however, and it was not until Sutter converted a Billy Carroll setup, after Ranger Peter Wallin inexplicably deserted both Carroll and the puck behind the Ranger net, that the sweep was assured.

It was only the third time in the Rangers' 55-year history that they were swept in four games, Montreal having accomplished it in 1967 and 1969. But, after all, this was a first-place team against one that finished 13th, and those first-placers were playing exceptional hockey.

"Our power play was great," Bossy said. "If they wanted to play physical, they knew our power play was going good. Our penalty killing was great, too. Overall, we just played super hockey."

The Islanders' two power-play goals gave them a playoff record of 26, one more than they scored a year ago. Bossy's pair brought him another record of eight, two more than the mark he shared with Andy Bathgate, Bobby Hull and Jacques Lemaire. The short-handed score enabled the Islanders to match their 1980 playoff record of seven.

"This series was won by the Islanders' special teams," Patrick said. "They just killed us with them. Their power play and penalty killing were excellent."