"A Valediction: Forbidding Mourning" is stuffed to the brim with literary devices, from the sound and rhythm of the poem to the way it represents ideas.
Form/rhyme scheme: This poem is a formal one, and has a set pattern. It is arranged in quatrains with pairs of rhyming couplets, with an ABAB rhyme scheme (eNotes "A Valediction: Forbidding Mourning" - Style). This means that the first and third lines of each stanza will rhyme, and the second and fourth will rhyme. See? (text of the poem available in eNotes "A Valediction: Forbidding Mourning" study guide)
(A) As virtuous men pass mildly away,
(B) And whisper to their souls to go,
(A) Whilst some of their sad friends do say, The
(B) breath goes now, and some say, No:
There's also a bit of hyperbole, or exaggeration to make a point--Donne tells his wife to create "No tear-floods, nor sigh-tempests." Of course, no one could cry a whole flood; he's just using exaggeration to show how sad she could appear. This particular hyperbole is also metaphor, comparing the tears and sighs of pining for a loved one to the storms of the world.
The metaphor of earth-moving storms becomes extended as Donne comforts his wife by arguing, logically, that the huge movements of the planets, which are much bigger, are no cause for alarm and therefore she should not be troubled by her comparatively small emotional turbulence: "But trepidation of the spheres,
Though greater far, is innocent." This type of intellectual argument to explain emotional matters is typical of metaphysical poetry, which "A Valediction" exemplifies.
Another device--which is also a metaphysical trademark--is Donne's use of metaphysical conceit. In Bussey's critical essay featured on the eNotes study guide, we learn that "[Metaphysical conceit] refers to a technique used by metaphysical poets in which commonplace objects or ideas are used to create analogies, offering insight into something important or profound." You can think of them as "special similies" that use comparison with everyday items to make a deep point. One of the most famous metaphysical conceits ever is the one that closes out "A Valediction"--that of the compass points representing two souls. (Keep in mind that he's referring to a drawing compass; you know, the one you use to draw circles in geometry?) Let's take a look:
If they be two, they are two so As stiff
twin compasses are two;
Thy soul, the fix'd foot, makes no show
To move, but doth, if th' other do.
And though it in the center sit,
Yet, when the other far doth roam,
It leans, and hearkens after it,
And grows erect, as that comes home.
Such wilt thou be to me, who must,
Like th' other foot, obliquely run;
Thy firmness makes my circle just,
And makes me end where I begun.
Here, Donne creates a beautiful comparison between the fixed, unmoving center point of the circle (his wife) and the wandering point that draws the circumference (Donne). Just like these compass points, Donne and his wife remain irremovably connected even as he moves away. He'll always return to where he began, linked to her always.