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 towards love in "The Broken Heart", is it okay to use only the first two stanzas?
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.
There are plenty of reasons to analyze just a portion of a work or art. In analyzing how a poem works, you simply analyze how the poem works in those two stanzas. Your approach determines whether or not your topic and subject are acceptable.
What are the effects of those two stanzas? How are the effects accomplished? That's what you analyze.
That said, you certainly do want to be aware of how thoses two stanzas contribute to the work as a whole. That's what makes the two stanzas worth analyzing. But it is entirely appropriate to center on just part of a work.
In fact, you could study a single line of a poem in an essay if it contains enough rhetorical material to study.
The answer to this question depends on your essay assignment. For example, sometimes you will get an assignment where your teacher wants a profoundly detailed and precise response on a particular segment of text. If you were to attempt this for a whole poem, you might end up writing a book! - particularly if the poem was a long one like William Wordsworth's '"The Prelude." It is usual though, when asked to look at themes or comment on a poem, to look at the work as a whole, because quite often the poet's message will be at the end. You may also have to detail techniques, some of which may be in the concluding stanzas. So, to play safe, unless the assignment asks for a specific piece of text - try to refer to the beginning, middle and end.
Yes, it is acceptable to use only part of a poem when, of course, your paper's thesis is supported when using only part of the material that you are writing about.
Poems must be quoted in a particular manner. For example, the first four lines of Spring By William Shakespeare would be quoted as such: “When daises pied, and violets blue,/ And lady-smocks all silver-white,/ And cuckoo-buds of yellow hue/ Do paint the meadows with delight.”
It should be perfectly fine to only use the first two stanzas because you are looking for something specific. In comparison, you would possibly use on a portion of Dante's Inferno for a paper. Hope this helps.