Since they were ambushed by the Buffalo Sabres in the semifinals of the 1975 Stanley Cup playoffs, the Montreal Canadiens have played 181 hockey games without losing two in a row.

So what must the New York Islanders do to reach the Stanley Cup final? They must defeat the Canadients three straight.

Game five of the best-of-seven semi-finals which Montreal leads, 3-1, will be played at the Forum Tuesday night. Canadians, during the last two years have lost only four of 91 on home ice. That static does not figure to increase the Islanders' staying power, either.

Still, not even Team U.S.A. would publiclly concede defeat before it met the Russians, and the Islanders, verbally at least, expect to overcome this monumental challenge.

"We've lost before and come back," said New York coach Al Arbour. "We're a professional hockey club and we scratch and claw for everything we get. I'm an optimist by nature and there could be a lot of hockey left. We have to look ahead with optimism."

Defenseman Denis Potvin, somewhat slowed by muscle spasms in his leg, said, "I think this is still far from over. My leg still hurts, but that's not an excuse. It felt the same way the other night when I had a pretty good game and we beat them."

The Islanders are the only team to win a playoff game from Montreal in two years, having won the fourth game of the 1976 semi-finals and the third game this year.

The Canadiens are hurting on defense, with Bill Nyrop in pain from torn chest muscles and Pierre Bouchard recovering from a shoulder separation victim. Obviously, the Canadiens are capable of plugging any holes that may develop.

Should the Islanders surprise, game six will be played at Nassau Coliseum Thursday night. Otherwise, the best-of-seven final with the Boston Bruins will open here Thursday. Regardless of the situation, Washington-area fans will finally be able to see a playoff game, since the Thursday contest will be shown on Capitol Centre's Tel-screen at 8 p.m.

The Bruins were grateful for the opportunity to nurse their wounds while awaiting the outcome of this series. Although free of major injuries, Boston had numerous bruises to assauge after its sweep of Philadelphia that concluded with a 3-0 decision in Boston Sunday night.

"We won't be unhappy waiting for a few days," said goalie Gerry Cheevers, the players' choice as No. 1 star. "We have some guys who use that time for rest. That includes me. TOu know I have to be a little sore after playing four times against a team like the Flyers this week."

"Who'd have thought we'd be resting up for the Canadiens?" asked winger Don Marcotte, whose accomplishment in holding the Flyers' Reggie Leach without a goal was one of the keys to the sweep. Another big defensive effort was supplied by center Gregg Sheppard, who kept Rick MacLeish, a 49-goal regular season scorer, off the board.