The opening lines of "When I have Fears That I May Cease to Be" establish the poem's primary thematic concerns: the fear of death and the cutting short of creative potential.
In the first line, the speaker describes moments when he or she fears dying. The line is enjambed, spilling over on to the next line and quickly establishing a sense of anxiety—of the thoughts in the speaker's mind surging forward of their own accord.
The second line is a continuation of the same sentence, and reveals that the speaker does not merely fear "ceas[ing] to be" but dying "Before my pen has gleaned my teeming brain" (emphasis added). In other words, it is not only a fear of death itself that fills the speaker, but a fear of dying before achieving something creative. In fact, the wording "cease to be" is significant, in that the speaker does not fear death itself, but a lack of life, and the possibilities therein.
Specifically, the concern over dying before the speaker's "pen has gleaned [his or her] teeming brain" implies that the speaker's primary interest is writing. The line features metaphor, describing the image of a pen gleaning the speaker's mind. Such an image likens a pen to a scythe, a tool uses to harvest crops. The metaphor suggests that the speaker wishes to harvest the crops of his or her "teeming brain," or, more literally, write down the speaker's many thoughts. The metaphor is especially complex in that the speaker is both the harvester and thing harvested.
Each line also begins with a word related to time: "When" and "Before." By placing these words at the start of each line, our attention is drawn to the temporal aspect of the poem, and gives rise to a sense of urgency in the speaker's fears. "When" suggests that the speaker has fears of dying on a regular basis, while "before" suggests that that particular fear revolves around dying too soon.
Furthermore, the lines rely on assonance, or the repetition of internal vowel sounds. "Fears," "cease," "be," 'before," "gleaned," and "teeming" each feature /ee/ sounds. Because the sound first appears in the word "fear," however, the sound's continued prevalence throughout the lines embodies the prevalence of the speaker's fear, making the word "fear" feel present in the poem even when it isn't directly.
Lastly, these lines establish the poem's meter. Written primarily in iambic pentameter, each of these lines features five perfect iambs, or unstressed-stressed beats, that add up to ten syllables. For example:
When I have fears that I may cease to be
Such perfect meter gives the impression of control, as if the speaker is able to tame his or her fear of death. Additionally, such meter was a common feature of Elizabethan sonnets, and may act as the speaker's method of gesturing toward other past writers, such as Shakespeare. Doing so might link the speaker's interest in writing with the great writing of other literary figures.