Oh, I just saw this post. Yes. If you're reading English or Italian poetry, you should be aware of the importance of the rhythm and length of the line, also known as its "meter." Meter is a complex issue, which I can't cover fully here, but I'll try to give a brief explanation.You mean the poet use apostrophes to make a setence rhythmic?
In English (and most European languages, including Italian) poets often count the number of syllables (let me know if you don't understand syllables--they are the parts of a word) in a line to make it come out right. The most common meter is iambic pentameter, which basically means ten syllables per line. Most commonly every other syllable is stressed, meaning that it is voiced more strongly, giving the line it's rhythm. Poets can play with where they put stressed syllables to make their poetry sound a certain way.
This may be a lot to take in. Don't worry too much about stresses for now. They may be hard to hear at first if you're not a native English speaker.
Let me start by showing you how the second line of the translation you're looking at looks broken into syllables (it is more regular than the first line and a better example for explanation):
Re-clin-ing, on the slen-der oat re-hearse
This line breaks up evenly into ten syllables, the most common line form in English, and every other syllable is stressed.
The first line is a little tricky because it actually breaks up into eleven syllables:
You, Tit-y-rus, 'neath a broad beech-can-o-py
The poet can get away with the eleven syllables because of the way the stresses are distributed, he's put in an extra unstressed syllable which sort of only partly counts and is allowable (don't worry too much if you don't follow the logic behind this).
The point is that, while the eleven syllable line is sometimes allowable, the poet really doesn't want a twelve syllable line here because it would sound akward (though there are other places where poets use twelve syllable lines) . That is why he takes out a syllable by taking the "be" off of "beneath."