The first two lines of “When I consider how my light is spent” establish the poem’s broad concern and its form. The speaker begins by complaining about losing “light,” or going blind. The loss of eyesight can be thought of as a physical symbol of a spiritual problem—how to best serve God—and the rest of the poem will be dedicated to working through this spiritual crisis. The second line of the poem amplifies the stakes: the speaker has gone blind and fallen into spiritual crisis, before even reaching middle age! The speaker might feel differently if this blindness had come later in life, after the speaker had accomplished more. As it is, the first two lines of the poem suggest that the speaker feels unable to use his or her capacities and talents to their full potential—a suggestion the speaker will explore in more detail in the following lines of the poem.
Just as the speaker opens in the poem in spiritual crisis, the poem itself is marked by formal tension and confusion. “When I consider...” is a Petrarchan sonnet. Like all Petrarchan sonnets, it uses just two rhyme sounds in its first eight lines, giving those lines an obsessive, churning feel: the speaker seems unable to escape from this doubt and anxiety, just as the speaker is unable to escape from the same, repetitive rhyme sounds.
Also like most sonnets in English, “When I consider...” is written in iambic pentameter: a meter the speaker handles easily and smoothly here, though the speaker will run into difficulties later in the poem. Moreover, the speaker here uses traditional literary devices like assonance (the /i/ sound in the first line) and alliteration (the /w/ sound at the end of line 2)—though the speaker will later largely strip the poem of those devices, favoring an unadorned (and, indeed, a more Puritan) style.
On the surface, then, “When I consider...” begins as a properly executed Petrarchan sonnet: nothing is obviously strange or amiss. Its formal disturbances are buried a bit, under the surface of the poem. For instance, the first line of the poem inaugurates a long sentence, which stretches until the middle of line 8. The sentence is unusually punctuated, but if one breaks it into its pieces, it has a clear conditional structure: “When I think about this… then I ask the following question.” Because of its conditional structure, and because the independent clause that completes the conditional clause is delayed for so long, virtually all of the poem’s first eight lines are arguably enjambed. (The exception is line 7, which is technically grammatically complete on its own and therefore end-stopped, even though it feels enjambed).
The result is a proliferation of caesuras: the poem’s phrases and clauses terminate in the middle of the line rather than the end. The speaker fails to calibrate the length of phrases to the length of the poem's lines, giving the poem a hectic, jerky feel. Though the poem may be a sonnet, its internal architecture reveals a speaker in crisis, unable to fully control his or her poem.
The initial reference to "light" may also be an allusion to the biblical Parable of the Foolish Virgins. This story is usually interpreted as a call to prepare for Judgment Day, i.e. to meet God. Like the Foolish Virgins, the speaker feels as though with the loss of sight he or she has lost the capacity to commune with God, to meet God as he offers salvation.