Answer: The structure of the poem is a ballad.
Explanation: Although it is difficult to analyze the structure of the poem in this format (it has not been divided in stanzas), it is possible to say that this poem is a ballad, specifically one of the so-called Child Ballads, anonymous and traditional ballads from England and Scotland that were compiled in the 19th century by American scholar Francis James Child. A ballad is, esentially, a narrative song, a poem meant to be accompanied by music. This one, which tells the story of how a Scotchman offered himself to be Robin Hood's servant, features seven four-line stanzas. In each stanza, only the second and the fourth verses rhyme:
Now bold Robin Hood to the north would go,
With valour and mickle might,
With sword by his side, which oft had been try'd,
To fight and recover his right.
The first that he met was a jolly stout Scot,
His servant he said he would be;
"No," quoth Robin Hood, "it cannot be good,
For thou wilt prove false unto me.