How are the first and last at line's of Langston Hughes poem different

The poem is made up of two quatrains (four-line stanzas). It follows the rhyming scheme of A-B-C-B in each stanza. The first line of both the stanzas is a refrain that emphasizes the theme of the poem. The first, second, and last line of each stanza consisting of four syllables. Whereas the third line consists of six syllables

Explanation:

imparadis't in one anothers arms  

the happier eden, shall enjoy their fill  

of bliss on bliss, while i to hell am thrust,  

where neither joy nor love, but fierce desire,  

among our other torments not the least,  

still unfulfill'd with pain of longing pines;  

yet let me not forget what i have gain'd  

from their own mouths; all is not theirs it seems:  

one fatal tree there stands of knowledge call'd,  

forbidden them to taste: knowledge forbidd'n?  

suspicious, reasonless. why should their lord  

envie them that? can it be sin to know,