Final answer:
The tone of Housman's poem is somber and reflective, with imagery that emphasizes the regret of giving one's heart away in vain and the resulting 'endless rue.'
Step-by-step explanation:
The tone and imagery in the bolded lines of A.E. Housman's poem "When I Was One-and-Twenty" convey a somber and reflective mood. The word "vain" suggests futility and pointlessness, carrying connotations of regret when giving one's heart away. "Endless rue" implies a perpetual state of sorrow and regret, amplifying the melancholic mood of the poem. The interjection "oh" at the beginning of the last line punctuates the realization of the speaker with a profound sense of weight and authenticity. This line combines with the others to evoke a feeling of deep regret and the pain of wisdom gained through experience.