Can anyone explain the themes of John Keats's poem "To Autumn"? Why did Keats write this poem and used many imagery, motifs from nature?. I...

In a letter to a friend named Reynolds, Keats explained that he composed "To Autumn" because

Somehow a stubble plain looks warm--in the same way that some pictures look warm--this struck me so much on my sunday's [sic] walk, that I composed upon it.

"To Autumn," the " perfect embodiment of poetic form, intent, and effect," is an ode, a serious and dignified lyric poem that adheres to a stanzaic form and is fairly long.  Keats's ode is divided into three eleven-line stanzas with the rhyme scheme of abab  cdecdde.  Autumn is personified by Keats, and he employs apostrophe, addressing Autumn as a woman:

Who hath not seen thee oft amid thy store?..../thee sitting careless on a granary floor,/Thy hair soft-lifted by the winnowing wind/....And sometimes like a gleaner thou dost keep/Steady thy laden head across a brook/Or by a cider-press, with patient look,/Thou watchest the last oozings hours by hours.

There is splendid imagery in this ode.  The "mellow fruitfulness" of the vines and apples, the "mossed cottage-trees," the "plump hazel shells," and "sweet kernel" along with "fume of poppies" suggest the sights and smells of the harvest of fall. In the third stanza as autumn nears its end, the "barred clouds bloom" and "touch the stubble-plains with rosy hue" while gnats mourn "in a wailful choir."  Other sound imagery is found in the bleating of the lambs, the singing of the crickes, the whistling of the "red-breast," and the twittering of the swallows. 

As the stanzas are arranged in the order of the progression of the season of autumn. The first stanza describes the early stages of the season with images of fruit ripening, the "maturing sun," the gourds, the vines, and the hazel nuts all ripen. In the second stanza, the ripening process of the fruit is complete and the fruit is stored as Autumn falls asleep.  But, she awakens to watch "the last oozings [of the cider-press] hours by hours.  It is as if Autumn wishes to capture Time and halt the coming of Winter. Here Keats expresses the theme of the transience of time.  Finally, in the third stanza, winter approaches.  When Keats asks Autumn

Where are the songs of spring?.../Think not of them, thou hast thy music too,--

he suggests that Autumn has her own beauty, albeit a passing beauty [theme].  And, again, the theme of the transience of time is implicit in the third stanza as Autum comes to completion:

Thn in a wailful choir the small gnats mourn/Among the river sallows, borne aloft/Or sinking as the light wind lives or dies;...The red -breast whistles from a garden-croft; And gathering swallows twitter in the skies.