The sonnet has a definite sense of strophic development, and the frequent twists' in the narration necessitate a close examination of this. The sonnet begins with a "When" clause, launching the reader on a sentence of indeterminate length and subsequently leaving us with expectation, in suspense, at the end of the line. The woman is emphatic: she does not merely tell the truth, she is made of truth. Both the nature of this truth, and the reason for her swearing it, are unknown to the reader. The immediate thought is that the speaker has challenged her in some way, and whether or not this is correct, it is certainly an unconventional way to begin a love poem.
The second line, "I do believe her, though I know she lies," introduces the reader to the wry humour that is an important feature of this sonnet. The humour is produced by the comic contradiction between outward behaviour (since the speaker's belief in her words is a reaction to her speech and thus a social act) and inward: his knowledge that she is lying. The narrator's calm tone evokes confusion: he is not angry with the woman, nor does he seem at all embarrassed to make such an illogical statement. The fact that he states "I do believe her," rather than simply "I believe her," combined with the caesura that follows this statement, serves to reinforce his belief in the eyes of the reader, though his reasons for this are as yet unclear. However, they are answered in the final lines of the stanza. When "That" is read as "so that," the reader learns that the speaker's own motivation is also one of deceit; he not only wants her to think of him as a "youth," but also to see him as simple, naïve, unsophisticated, "untutored." At the same time as he is deceiving himself by believing her lies, he is in effect mirroring her actions. He presents himself as "made of truth" by establishing himself as an innocent, "Unlearnèd in the world's false subtleties."
Shakespeare begins the second stanza with a wonderful pun. "Vainly thinking" refers not only to the narrator's own vanity (which is driving him to such a deception), but also to the futility of his efforts. He realises that, though he hopes she will deceive herself into thinking him young and "unlearnèd," his efforts are in vain, for she knows the truth about both his age and his experience. Another richly complicated word here is "Simply," which begins the third line of this stanza. Given the paradoxes of the previous lines, it is
probably the last word one might have expected here: it begs the question, is it at all simple to believe a liar?' The word is additionally reinforced because its meter shows the only deviation from iambic pentameter in the whole poem. Instead there is a spondee, forcing the reader to pay more attention to what is meant by the use of...