The Air Line Pilots Association and United Airlines reached tentative agreement last night on the major contractual dispute that caused the eight-day-old strike against the nation's largest carrier, but other matters threatened to keep the strike alive.

Helen M. Witt, chairman of the National Mediation Board, said that negotiators "have resolved all the contractual issues," but that difficult "back-to-work" questions remained. "I hope with further work in this regard, I'll be able to give you better news in the future," she said.

Witt has been keeping negotiators on a 16-hours-a-day schedule since Monday, when talks resumed "at her insistence," in the words of a United negotiator. Negotiations continued last night at a suburban Chicago hotel.

David Pringle, United's chief negotiator, told reporters that "the company's position is, if we don't get an agreement tonight, we will break off talks indefinitely."

There was no indication how quickly United could return to full service after a settlement. "That would depend on where the airplanes and crews are and what's economically feasible for us to do," airline spokesman Chuck Novak said.

United has been operating about 14 percent of its normal schedule of 1,500 daily flights since the strike began May 17, forcing thousands of passengers to scurry for other transportation, and dramatically increasing the loads carried by United's primary competitors, American, Continental, Western and Frontier.

When negotiations resumed Monday, only one issue separated the parties: United's intention to pay newly hired pilots less than current crew members and thus create a "two-tier" pay schedule.

Witt said that "the two-tier question is behind us," but declined to release details. The United pilots' Master Executive Council would have to give final approval to a negotiating committee agreement.

Ralph Colliander, a staff mediator with the board, said there is "intense bargaining under way on back-to-work issues," which he would not specify.

Such issues typically include management demands that the union take no action against members who crossed the picket line, and union demands that management not retaliate against strikers.

A potential significant complication is the status of 500 pilots United trained but did not hire. When the strike began, United offered them jobs, but only four came to work. The pilots union promised to protect those jobs, even though the question of whether it could represent unhired pilots is an interesting legal one.

Complicating the resolution of that issue were the public positions taken by both sides. United Chairman Richard J. Ferris said in an interview May 13 that the 500 new pilots "will be offered a job and either they come to work on the appointed day or you'll never work here, because they're not on the seniority list. Okay?"

And Henry Duffy, national president of the ALPA, said yesterday afternoon that "there isn't going to be any settlement without us protecting them. We signed them all up as ALPA members."

United announced yesterday that it had hired 65 replacement pilots, and the company would be obliged to protect them and their position on the union seniority list.

When negotiations began Monday, United's two-tier proposal would have cost the average new-hire pilot about $500,000 over a 30-year career, creating second-class citizenship and dissension in the cockpit, the ALPA contended.

United noted that a two-tier schedule would not lower the salaries of existing crew members and was necessary to make the airline competitive with American, the nation's second-largest airline. American has been adding service rapidly since it negotiated a two-tier contract with its pilots, who are represented by a separate union, in 1983.

Further, United said, low-cost airlines such as Continental and People Express pay flight crews about half as much as United's average cockpit salary of $80,000 to $90,000. Further, United has negotiated two-tier pacts with flight attendants and mechanics.