John Donne's poem, "A Valediction: Forbidding Mourning," is a lovely love poem about two lovers parting. The use of the word "mourning" may lead the reader to think of death, however research indicates that Donne wrote the poem as he prepared to go on a trip, and perhaps he refers to his wife's "mourning" his absence while she remains at home. This is, then, the subject of the poem: asking that the other who is left behind remembers not to mourn—for those who love one another are always connected, wherever they may "roam."
The first stanza does refer to death: of a noble man who has led a "good" life. Knowing that he have done well, he does not fear death, but looks forward to his "reward." He passes away so gently and calmly that those with him are not all certain that he is truly dead.
Whilst some of their sad friends do say,
"Now his breath goes," and some say, "No."
Death is painted in a positive light here: as something to be wished for by those who can face God without the regret of a wasted life. (As a metaphysical poet, Donne would also have written about one's relationship with God, as well as love.)
In eNotes.com, this poem is described as a conceit, which is an extended metaphor, or comparison. This is not apparent, however, until the second stanza; the first thing being compared in the first stanza (a dying man), is ultimately compared to (two lovers being separated) in the second stanza.
The quiet death of the man described in stanza one is now used as an example for those separated from a lover: they should act as mildly as the dying man. The eNotes.com summary suggests:
[Donne} might also be suggesting that their separation, though only temporary, will be like a small death to him.
But he asks his wife to stand strong and not take it hard, by making a scene...
No tear-floods, nor sigh-tempests move...
...which would make the separation more difficult. He explains to his wife that they are joined and are like one person. (This is an Elizabethan concept: that when a couple married, they were no longer separate entities, but merged into one person, the same flesh and blood.) Like one foot placed next to the other, if the first foot moves, the second will certainly have to follow the first. And no matter how they are separated, Donne knows he will always find his way back to his wife.
Donne's unusual images include the death of a nobleman, an earthquake and the moving apart of planets. There is also the image of the compass.
The earthquake's image presents an event that causes great fear:
Moving of th' earth brings harms and fears ;
Men reckon what it did, and meant ;
However, when planets move, people don't worry about it because the planets are so distant.
But trepidation of the spheres,
Though greater far, is innocent.
So his wife should respond as if they were planets rather than earthquakes: take not so much notice when the speaker leaves.
The image of the compass describes that it has two parts and needs both to work together collectively in order for the compass to do its job.
These images may seem unusual because they are not standard fare in poetry: we might expect more images of nature, and even love, but not of a dying man or a compass. These items all seem out of place in a poem from one lover to another.
Additional source: http://www.luminarium.org/sevenlit/donne/mourning.php