What is Shelley's view of the skylark's song in "To A Skylark"?

Shelley is struck by the beauty of the skylark's song, and attributes it to the fact that the bird soars high above the earth (so high, indeed, that he remains "unseen"), presumably with none of the cares that humans have to deal with. Shelley imagines that the bird's life must be idyllic, soaring through clouds and sunlight, and muses that if he could experience the same joy in his life, his poems might be filled with the same beauty that he hears in the skylark's song:

Teach me half the gladness/That thy brain must know; Such harmonious madness/From my lips would flow,/The world should listen then, as I am listening now.

Ultimately, then, Shelley uses the skylark's song to suggest that true poetic beauty is attained when the poet looks to nature, and frees himself from worldly concerns. It is thus both a wistful poem and one which articulates one of the core principles of the English Romantic poets. 

How does Shelley personify the skylark in "To a Skylark"?

One way in which Shelley personifies the skylark in the poem "To a Skylark" is to compare the bird to characters within the poem, like a "poet hidden" and a "high born maiden;" through his use of simile, the skylark is given human qualities (36, 41). Shelley suggests that the skylark uses his music to inspire and "soothe."  He also assigns an emotional quality to the bird as well as sensitivity and understanding by portraying the skylark as being so joyful that he must not know "languor" or "annoyance" (77-78).  In the final stanza, the speaker perceives the skylark as having knowledge worth sharing, and implores him to "teach me half the gladness thy brain must know;" projecting a deeper value and lesson into the skylark's singing that may not actually be there (101-102). 

How does Shelley relate the skylark's song to his poetry in "To a Skylark?"

The skylark is famous for becoming a symbol in Romanticism of beauty, eternity and understanding, amongst other things. In this famous poem, having tried to capture the bird's song and describe what it is like in vain, Shelley realises the futility of his task because none of the images he devises, such as comparing the bird to a rainbow cloud or a glowworm is sufficient to convey the sheer, ecstatic joy that the speaker feels when he listens to the skylark's song.

It is this joy that the speaker wants to learn or understand, because the skylark's joy is different from the joy felt by humans, whose understanding of joy is marred by the suffering we undergo. Learning how to capture such joy will enable the speaker to incorporate such a feeling into his poetry, radicalising the lives of his audience and benefiting his race. Note the plea in the last stanza of this poem:

Teach me half the gladness
That thy brain must know,
Such harmonious madness
From my lips would flow
The world should listen then--as I am
listening now.

Shelley thus recognises what is special about the skylark's song and speculates on what uniting this specialness with his poetic talent would achieve. Being aware of the power in nature and incorporating that into our frames is an immensly potent force, Shelley seems to suggest.

What makes Shelley's "To a Skylark" beautiful?

It is notoriously difficult for people to agree on what is or isn't beautiful. This doesn't necessarily mean that beauty is subjective, but there's no doubt that universal assent to the beauty to a particular feature of the natural world or a specific work of art is impossible.

Even so, successive generations of poetry lovers have agreed with each other that Shelley's "To a Skylark" is indeed a very beautiful poem, one of the most beautiful in the English language.

Primarily, this appears to be because people instinctively respond to the extraordinary richness of Shelley's language, a notable feature of his work. In "To a Skylark," for example, we have the luscious words of the fourth stanza, as good as anything Shelley ever wrote:

The pale purple even
Melts around thy flight;
Like a star of Heaven,
In the broad day-light
Thou art unseen, but yet I hear thy shrill delight.

Like his fellow Romantic poet Keats, Shelley had an instinctive feel for the sensuous, from which he was able to construct such remarkable works of poetry.

And his deep sensuousness conjures up remarkable images, as in the stanza we've just looked at. One can easily imagine the "pale purple" of the sky melting into the twilight around the skylark as he continues his majestic flight.

In this extract, we can see Shelley depict the beauties of nature as a whole as well as the profound effect that this remarkable bird has upon his imagination. In time-honored Romantic fashion, Shelley operates on the assumption that everything in nature is unified, joined together in a comprehensible whole. Shelley doesn't simply want us to understand this, he wants us to feel it too, which is why he overwhelms our senses with such lush, sensuous, and beautiful language.