The first four lines of “Sonnet 138” establish the poem’s theme and its form. The poem begins with a strange paradox. Whenever the speaker's mistress swears that she is “made of truth” (that is, metaphorically, whenever she swears that she is honest and faithful to him) the speaker believes her. Then, after a caesura, he announces an unexpected qualification: “though I know she lies.”
The speaker believes his mistress is faithful—and, at the same time, knows that she is lying to him. The caesura in line 2 thus seems like a wall within the speaker, which separates two contradictory beliefs. That contradiction is heightened by the alliteration that links together “love” and “lies.” The alliteration binds together the two words and suggests that, for this speaker, love is inextricable from lying.
This is a strange and puzzling opening to the poem. The reader might wonder how the speaker can possibly believe two things which are so obviously in contradiction with each other. But lines 3-4 begin to suggest an answer. The speaker is simply acting like he believes his mistress. And he does so because he hopes that she will think that he is an “untutored youth”—in other words, a naïve and inexperienced youngster.
The assonant /oo/ sound in “untutored” and “youth” suggests the connection the speaker is trying to make: the “untutored” are, it seems, young by definition. So if the speaker is “untutored”—if he doesn’t understand the “world’s false subtleties,” its tricks and dishonesties—then he must be young. Both the speaker and his mistress and therefore being dishonest and deceptive. She is trying to deceive him about her infidelities, while he is trying to deceive her about his age.
“Sonnet 138” is a Shakespearean sonnet, a demanding kind of formal verse. Though Shakespeare did not invent the form that bears his name, he did master it—and, eventually, popularize it. Sonnet 138 displays that mastery. It is written in strong, fluid, and highly regular iambic pentameter. And it closely follows the sonnet’s ABAB rhyme scheme, using strong, straightforward rhymes. Notably, all of the poem’s lines are end-stopped, which gives it a slow, heavy feeling. The speaker is so troubled by his complex, demanding relationship that he seems to struggle to keep the poem itself going.