By John Beaufort

The Roundabout Theatre Company is celebrating the 20th anniversary of its founding with a good-natured revival of a sturdy 18th-century comedy classic. Oliver Goldsmith's ''She Stoops to Conquer'' was originally entitled, ''The Mistakes of a Night,'' which became its subtitle. The series of mistakes and mistaken identities generates the amusing mix-ups that take place in the English countryside a little over 50 miles west of London in September of 1744.

The mistakes begin when young Marlow (Norman Show) is duped into believing that the home of his designated father-in-law, Squire Hardcastle (E. G. Marshall), is a wayside inn. The cause of the young man's unaccountable behavior is known only to Hardcastle's stepson, the rascally Tony Lumpkin (Nathan Lane), who perpetrates the hoax but who, in the end, proves to be more of a friendly conspirator than a lumpish bumpkin.

The animated performance, directed by Daniel Gerroll, had not yet quite achieved an easy ensemble at the preview I attended. But there was no mistaking its comic intent. The attractive cast stars Mr. Marshall as the nonplused and outraged Hardcastle, Tovah Feldshuh as an entrancing Kate, and Kaye Ballard as the affectedly voluble Mrs. Hardcastle.

Mr. Lane sees Tony as something slightly more than merely one of the brattiest brats in English comic stage literature. Along with his pranks and werewolf cry, there is a certain style and wit to his mischiefmaking. John Bedford-Lloyd and Cynthia Dozier make an attractive twosome as the secondary lovers of the comedy.

Set designer Robert Thayer has made a good effort to adapt the Triplex's awkwardly wide stage to the needs of the changing scene. But surely the Roundabout company could have afforded more than a single chair to furnish the Hardcastles' drawing room. Eloise Lunde created the picturesque period costumes and Allen Lee Hughes lighted the production. (The Triplex is the company's temporary home on the campus of Manhattan Community College/City University of New York, on Chambers Street, in the city's deep south. ''She Stoops to Conquer'' runs through Nov. 11.)

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