The first thing you need to cover in a critical appreciation of the poem 'The Darkling Thrush' by the poet Thomas Hardy is the poetic form, including both meter and rhyme scheme. The meter of the poem is alternating lines of iambic tetrameter and iambic trimeter, organized into eight-line stanzas rhymed ABABCDCD. These eight lines stanzas can be considered as consisting of two quatrains in common, or ballad, measure. The temporal setting of the poem, the dark day in winter, is also important because after the longest night, daylight hours start to increase. Next, you should look at the symbolic function of the thrush as it relates to the theme of hope despite loss of faith, as suggested by the final lines:
That I could think there trembled through
His happy good-night air
Some blessed Hope, whereof he knew
And I was unaware.