Final answer:
Alliteration is the poetic element that distinguishes Old English poetry from modern poetry, providing structure and a musical quality to the verse. Rhyme, meter, and simile are also poetic elements but were not predominant in Old English poetry.
Step-by-step explanation:
The poetic element that holds Old English poetry together and separates it from modern poetry is alliteration. Unlike modern poetry, which often relies on rhyme and meter, Old English poetry was characterized by the use of alliteration, where the initial consonant sounds are repeated in a line or in closely connected words. Alliteration was used to give structure, rhythm, and a musical quality to the poetry, making it easier to recite or sing. Examples of Old English poetry, such as Beowulf, clearly demonstrate the use of alliterative verse, an essential feature of the genre that distinguishes it from many forms of modern poetry that typically use rhyme as a key structure.
Rhyme, meter, and simile are also elements of poetry, but they do not hold the same foundational place in Old English poetry as alliteration does. Rhyme, for example, is more commonly associated with later periods of English literature.