Final answer:
The poem expressing the poet's fears about death and the brevity of life is "Ode to a Nightingale" by John Keats, in which the poet conveys a longing to escape life's sorrow and mortality through the eternal beauty of the nightingale's song.
Step-by-step explanation:
The poem which expressed the poet's fears about death and the brevity of life is "Ode to a Nightingale" by John Keats. In this poem, Keats conveys a sense of the transcendent beauty of the nightingale's song, which contrasts with the human fear of mortality and the longing for escape from the pains of life. The poet envies the nightingale's seemingly eternal existence and wishes to fade away with it, to forget the sorrows and the inevitable fate of death that all humans face. John Keats' other works also explore similar themes, including the praised "Ode on a Grecian Urn" and "To Autumn," but they do not focus as intently on the poet's personal fear of death.
Although not the correct answer, it is interesting to note that other poems mentioned, like "To His Coy Mistress" by Andrew Marvell and "To the Virgins, to Make Much of Time" by Robert Herrick, engage with the theme of carpe diem - seizing the day - emphasizing the brevity of life and the urgency of living fully before death arrives. However, these do not express the same kind of personal fear of death found in "Ode to a Nightingale."