To answer this question, it is first necessary to identify the topic of the poem "If no one ever marries me." This appears to be a simple matter, as the speaker is discussing how they will respond if no one ever marries them. However, it is difficult to see how such a specific topic could be conveyed by the form, setting, tone, or imagery.
A more general topic is "the world viewed through the eyes of a child." The form conveys this topic through its simplicity. The rhyme scheme is as simple as a rhyme scheme can be, abcb, and the meter is iambic tetrameter alternated with iambic trimeter. These short, regular lines give the poem the sound of a nursery rhyme or a children's song. The tone is equally simple, with all the certitude of childhood.
Identifying the setting may be the most difficult, since it is not mentioned explicitly. However, the presence of the nurse in the first stanza and the images which occur in the speaker's mind suggest a Victorian or Edwardian nursery. This locates the poem in a time and place when children were the objects of unprecedented sentimentality.
The setting is complemented and established by the imagery. The speaker sees a wife or husband as being much the same as a pet squirrel, rabbit, pony, or lamb. Twenty-nine seems an advanced age to them, and the procedure for adopting an orphan is no more complex or serious than that of buying a rabbit. The simple images of animals and people establish the youth and innocence of the speaker, connecting imagery with tone.