In his first concert with the National Symphony last night, Emil Tchakarov made it easy to understand how he won the 1972 Karajan International Conducting Competition. Since that time, he has been Karajan's assistant with the Berlin Philharmonic. He also is slated to make his debut with the Metropolitan Opera next season -- all this at age 30.

There was another youngster on hand for the concert: Steven de Groote from South Africa, the winner of the 1978 Cliburn Competition in Fort Worth, who is not yet 30. Between them they made the Chopin E Minor Piano Concerto a thing full of marvels.

The emphasis was on the concerto's heroic aspects, with strongly accented rhythms and powerfull attacks from the piano. But there was poetry, too, both in the outer movements and, in glorious ways, in the slow movement. Frequently the work touched heights, thanks to the way conductor and soloist dwelt for a moment on a change of key or a minute adjustment in tempo.

De Groote made exquisite sounds in the filigree of descending thirds, and, in the final pages of the larghetto, evoked from his big Bosendorfer some of the loveliest tones to come from any piano in that hall.

Tchakarov opened the evening with a plangent Jewish Poem by Bulgaria's leading composer, Pantcho Vladigerov, who would have been 80 yesterday. He died last September.

The music awakes memories of Wagner and Mahler, but has sounds all its own. Tchakarov led his fellow countryman's work with affectionate power, rewarded with beautiful playing from the orchestra.

The Dvorak G Major Symphony came off with the kind of dramatic sweep and tension that proclaimed its right to be thought the greatest of his symphonies. Tchakarov knows the value of waiting in music as well as the effect of exuberant rushing ahead. He and the orchestra played with grand effect.The concert will be repeated tonight and Thursday.