Brooke uses a number of literary devices in this poem. The first is his choice of a formal structure. This is a sonnet. Once readers recognize this, they can expect a number of other literary devices or specific details to be found in the poem. Standard sonnets have 14 lines, as this poem does. There are a few standard variations on sonnet structure. One variation is the Italian or Petrarchan sonnet. In this form, the first eight lines form one unit (the "octave" or "octet") and the final six lines form a second unit (the "sextet"). So, before you look further, look at this larger structure, and the relationship between these two parts.
After that, look for a regular rhyme scheme ("me" rhymes with "be," "field" with "concealed," and so on): ABABCDCD, etc. There is a regular rhythm. The lines are a specific length: ten syllables, usually in iambic pentameter.
Once you move past structure, there are other devices used. Brooke repeats words within lines ("rich"/"richer") or within the poem ("England"). He personifies the world ("blest by suns of home"). There is imagery throughout.
This intensely patriotic poem uses alliteration extensively to communicate the smooth-sounding rhythm and flowing verse that helps focus the reader on the image of a soldier setting out to possibly die but focusing on the way that his death would make a corner of the battlefield "for ever England" through his death. Note the following example of alliteration:
A body of England's, breathing English air,
Washed by the rivers, blest by suns of home.
The repetition of the "b" sound in these lines adds a sonorous tone to the sonnet as a whole, which is also emphasised through the alliteration of other sounds such as "s," "r," "d," "l" and "h." This, combined with the repetition of "England" and "English" places an emphasis on the theme of patriotism in the poem. Such devices show the extent to which this sonnet is a highly crafted set of iambic pentameters with a smooth, flowing rhythm. The almost paradisical images of England conjures up pictures of a beautiful countryside that ignores the urban reality of so much of Britain and perhaps stand in contrast to the mechanical nightmare that was captured in World War I through the use of technologically sophisticated machinery such as machine guns and barbed wire, which resulted in so much death. It represents a retreated to a deliberately more simple existence.