Is it okay to use only a part of a poem when you are writing an essay about it?for example, if you were to write about John Donne's attitude...

If you are being asked to write about John Donne's
attitude toward love in the above referenced poem, that sounds like the prompt is asking
you to look at a theme. Theme, tone, voice, mood, and author's purpose are all related
to your task and generally do take shape throughout a
piece.

I agree that if citing lines or using examples from
the poem you should carefully watch how this changes over the course of the piece and
pick quotes from the beginning, middle, and end. Often, a lesson learned and displayed
through a work like that is presented in one light in the beginning, but changes by the
end.

If after reading the entire poem you find there is no
change in attitude and the best quotes or references come from the beginning, I would
use those.