Explain why the poet structure the poem brennan on the moor into stanzas of four and two lines

"Brennan on the Moor" is structured in this way, so that the longer stanzas can follow the tale of Brennan and give more details to the story. The shorter stanzas act as a chorus, which is usually used to come back to the main idea of a song or poem (in this case, the idea that Brennan is the hero of the poem).