Love's not Time's fool, though rosy lips and cheeks Within his bending sickle's compass come; Love alters not with his brief hours and weeks, But bears it out even to the edge of doom. In Sonnet 116 by William Shakespeare, what text structure of a sonnet do these lines illustrate

I would say these lines illustrate a quatrain.

A quatrain is a stanza consisting of four lines, as is the case here. Shakespeare's sonnets have a specific structure - the first twelve lines are divided into three quatrains (4*3 = 12); the final two lines are called a couplet, and the message of the entire sonnet can be found in those two final lines.