Poetry is scanned, that is, read metrically, in units of feet. The basic feet are:
IAMBIC: two-beat, rising rhythm, as in adVANCE
TROCHAIC: two-beat, falling rhythm, as in FALLing
ANAPESTIC: three-beat, rising rhythm, as in holoCAUST
DACTYLLIC: three-beat, falling rhythm, as in FRIGHTening--not very common in English
SPONDAIC: two-beat, with both beats stressed--uncommon.
English is a stressed language; that is, meaning is conveyed in part by the contrasting accents of syllables, as in "dessert" versus "desert". Some theories of poetry consider number of syllables as well ("accentual syllabic" scansion), but for most purposes it seems best to consider the number and placement of stressed or accented syllables. The essence of poetry is variance within repetition, so you should look for broad patterns, and whenever possible try to interpret seeming exceptions as variations of an underlying rhythm rather than entirely new beginnings. (For example, a poem may be basically trochaic tetrameter with iambic passages, or iambic tetrameter with anapestic variations in the second and third lines of each quatrain.)
Of course there may be differences in how individual readers will inflect a series of words or a line, especially in the apportionment of SECONDARY STRESSES, but most of the poems we read have a clear identifiable metrical form. When learning to scan, read a couple lines out loud until you start to hear a consistent pattern. After you have the basic pattern, note how variations are used to extend the author's theme.
(Dunbar) Not they who soar, but they who plod
Their rugged way, unhelped, to God. . . .
And ever the man he rides me hard,
And never a night stays he,
For I feel his curse as a haunted bough,
On the trunk of a haunted tree.
ballad stanza, with two anapests in the third and fourth lines
(Spencer) Ah, how poets sing and die!
Make one song and Heaven takes it;
Have one heart and Beauty breaks it;
Chatterton, Shelley, Keats and I--
Ah, how poets sing and die!
(First foot of third line is dactyllic.)
Lady, Lady, I saw your hands,
Twisted, awry, like crumpled roots,
Bleached poor white in a sudsy tub,
Wrinkled and drawn from your rub-a-dub.
(The second and fourth lines switch to iambic after an initial trochee.)
Also see Dunbar's "An Ante-Bellum Sermon"
Dunbar's "Sympathy": trochaic and anapestic tetrameter (with a pentameter first line in each stanza)
Anapestic and trochaic Dactyllic tetrameter:
(Fauset) What is the change that creeps sharp over you?
Just as you raise your fine hand to my hair,
Bringing that glance of mixed wonder and rue?
(The terminal unstressed syllables are dropped, giving a syncopated effect.)
FREE VERSE: highly variable line lengths, no rhyme, carefully balanced clauses, repetition, alliteration, and assonance.
Examples: Fenton Johnson's "Tired" and "The Scarlet Woman."