Ten days ago Sir Peter Pears was singing in Benjamin Britten's War Requiem with the National Symphony, in performances that left indelible memories with those fortunate enough to hear them. This week he will give three master classes at Maryland University and join in a recital there on Tuesday night of music by Purcell, Schubert and Britten.

Through his lifelong association with the late Lord Benjamin Britten, Sir Peter has helped to change the face of 20th cantury music, an apportunity that is given to few musicians. Yet that is precisely what Sir Peter achieved through the interpretive power and insights of his musicianly art.

A quick look back at several instances that parallel his contributions to music will prove the point. Franz Schubert would certainly have written songs even if he had never met Johann Michael Vogl. But it was Vogl's art as well as his ability to promote Schubert's songs in influential Viennese circles that led Schubert to some of his greatest writing.

It is less certain that Mozart would have written his ineffable quintet for clarinet and strings, as well as the clarinet concerto and trio if he had not heard Anton Stadler playing on the new instrument in a manner that brought the new instrument to Mozart's attention in a new light.

And while the clarinet was firmly established a century after Mozart's time, it was not until Brahms was deeply moved by the playing of Richard Muhlfeld that he, like Mozart, created a clarinet quintet and trio as well as two sonatas whose pages proclaim the composer's admiration for the interpretive artist.

Consider, then, the music that Benjamin Britten wrote for the man whose consummate art was Britten's principal inspiration for nearly 40 years. It was for the unique sound of Peter Pears' voice that Britten shaped the great scenes in "Peter Grimes" that Pears first made world-famous: "What harbour shelters peace? Away from tidal waves, away from storms...." And later, "Now the great Bear and Pleiades where earth moves are drawing up the clouds of human grief grief, breathing solemnity in the deep night."

Britten's music in these passages is what it is because of the composer's intimate knowledge of every nuance of which Pears' inimitable voice was capable. Bearly 30 years later that knowledge had only deepened as the result of decades of affection and admiration during which the two musicians, creator and interpretor, joined to bring to the world's concert and opera stages some of the most affecting music of the century.

Those decades saw the creation of a dozen Britten operas fashioned around the musical personality of Pears. From the tragedy of "Peter Grimes" these moved on to the mystical symbolism of "The Rape of Lucretia," in which Pears was unforgettable as the Male Chorus; "Albert Herring," a comedy with Pears as the central figure in an adaptation of the Maupassant story.

And what about the evil mystery in the character of Peter Quint, taken from Henry James' story, a figure to which Pears brought the strong aura of terror? For Herman Melville's "Billy Budd," Britten created one of the strongest roles in the entire Pears roster, that Captain Edward Fairfax "Starry" Vere. As these operas followed one another, Pears was the moving force in making clear, through his abilities as singer and actor, that unbroken thread that runs through Britten's works, the thread of compassion for human miseries, in whatever forms they appear.

Returning in various guises, this thread kept returning Pears to the stage in "A Midsummer Night's Dream," to television and later the atage in "Owen Wingrave," and finally to the towering part of Aschenbach in "Death in Venice."

It was after Pears' debut at the Metropolitan Opera in 1974, at the age of 64, in the role of Aschenbach, that a New York critic wrote, "Ther is the amazing Peter Pears -- at 64, his voice richer and more secure an instrument than it sounded decades ago, his command of English diction a thing of beauty that no singer present or future can fail to profit from, his stage presence commanding with the slightest movement or gesture." It would be difficult to sum up Pear's art as an opera singer more succinctly or accurately.

Yet Pears the opera artist is only a part of this extraordinary musician. To an entire generation of record collectors, it was the famous recording of Britten's Serenade for tenor, horn and strings that first made the name of Pears an object of astounded admiration.

With Dennis Brain's phenomenal horn playing, Britten's strings introduced Pears' voice in a phrase that haunts anyone who heard that early London "ffrr" recording. Even subsequent recordings by Pears have not effaced the memory of that electrifying singing.

That "Serenade" was only one of a dozen major works Britten wrote around the Pears abilities. The Michelangelo Sonnets and those of John Donne, the Spring Symphony and the Cantata Misericordium, the Canticles and the popular Christmas story of St. Nicholas, all came into being because of Pears. And Washington, and a small part of New York, together with those who know the recording, will always hold in memory the impact of Pears upon the words and the music of the War Requiem.

But no great artist achieves his greatness by being a one-composer performer. Peter Pears started out early in becoming the musician who could eventually inspire creative musicians. He played the piano well enough in the Schumann Quintet to cause his school to start an orchestra. Not long after that he became organist of Hertford College at Oxford, before beginning to study singing with teachers and coaches who included Elean Gerhardt and Therese Schnabel, the wife of the great pianist.

His opera roles include Mozart's Tamino and Ferrando, Rossini's Almaviva, Verdi's Duke, in "Rigoletto," and Alfredo in "La Traviata," David in "Meistersinger," Vasek in "The Bartered Bride," and a superb Pandarus in Walton's "Troilus and Cressida."

In oratorio he was held to be incomparable as the Evangelist in the Bach Passions after World War II to such an extent that when soloists were being chosen for Otto Klemperer's towering recording of the St. Matthew Passion, EMI "borrowed" Pears from Decca because they regarded him as the only possible Evangelist imaginable as a colleague for Schwarzkopf, Ludwig, Fischer -Dieskau, Gedda and Berry. He is as notable in "L'enfance du Christ" by Berlioz, and matchless in Elgar's "Dueam of Gerontius," under Britten's baton.

As the world of song literature, it is generally conceded that three men are today's supreme interpretors of Schubert's "Winterreise" and Schumann's "Dichterliebe." Alphabetically they are Fischer-Dieskau, Pears and Souzay, accompanied by Gerald Moore, Britten and Dalton Baldwin.

And, just as Gerald Moore once said, "Why should I go to Aldeburgh (the site of the music festivals founded in 1948 by Britten and Pears) since the world's best accompanist is already there," so it is widely agreed that there are elements in Pears' singing of these songs that are incomparable.

Having been rewarded by his singing in the War Requiem in three performances two weeks ago, Washington is now, thanks to the University of Maryland, about to enjoy Pears as teacher and singer at classes on Monday and Tuesday and a concert on Tuesday evening. Does this remarkable 68-year-old plan to reduce his present schedule in the near future? He says:

"As I see it now, I'll go on singing at approximately the same pace -- which is plenty fast enough -- for about two more years. In 1980, when I shall, God willing, be 70 years old, I will cut down all foreign travel. Primarily, I intend to teach at the Britten-Pears School." Nothing could be more appropriate.