Patterns in poetry are often established in order to break them-- in the same poem. For example, in Richard, Duke of York, Shakespeare uses this to good effect:
So many hours must I take my rest,
So many hours must I contemplate,
So many hours must I sport myself,
So many days my ewes have been with young,
So many weeks ere the poor fools will ean,
So many years ere I shall shear the fleece.
So minutes, hours days, weeks, months, and years,
Passed over to the end they were created,
Would bring white hairs unto a quiet grave
Here, Shakespeare does more than just set up a pattern with "so many", but also with a progression of time, from minutes to hour to hours to days, etc... When it returns to minutes, this is unexpected, and the line is highlighted.
This can also be done in metre. A poem with iambic metre might go like this:
my sweet Britannia
is my dear sweet homeland.
Here, the first two lines establish a pattern of iambic rhythm. In the third line, this serves to highlight the word "my" over the word "dear", when the stresses were the other way around in the first line.
But sometimes, a poet does something unusual, in which our tendency to read the line might actually go against the tendencies both of the metre and the natural rhythm of the phrase. Consider, for example, this excerpt from "An Irish Airman Foresees His Death," by Yeats...
Those that I guard I do not love;
My country is Kiltartan Cross,
My countrymen Kiltartan's poor,
The first two lines begin with stressed syllables, which creates a tension in those lines, since the poem begins somewhat with a solid iambic tetrameter. But what about the second two lines of this excerpt? Should the emphasis be placed on "my" our on "country" ? If you follow the pattern which Yeats established in the first two lines, the emphasis should fall on "my". But if you follow the tendencies of the [English] Language, the emphasis should be on Country. This is precisely the dilemma which Yeats is highlighting in the poem; to the discerning reader, the same dilemma is present in the language, in the metre itself as in the plain statements of the poem.