“canker in the fragrant rose, Doth spot the beauty”
A 'canker' is a plant disease — a spreading rot or blight that eats into stems and buds from within. Here the youth's shame is the hidden corruption inside an otherwise beautiful flower: the rose still looks and smells lovely to the world, but the rot is real. 'Spot' means to blemish or stain.
- TLN 1325rhetorical device
“what a mansion have those vices got, Which for their habitation chose out thee”
The poet personifies the youth's vices as tenants who have found an extraordinarily grand lodging. A 'mansion' is simply a large, fine dwelling place — the youth's beauty is so magnificent a house that even vices are elevated by living in it. 'Habitation' means dwelling or place of residence. The device is ironic: the vices are lucky to have found such distinguished quarters.
- TLN 1327rhetorical device
“beauty's veil doth cover every blot, And all things turns to fair that eyes can see”
This is the central conceit of the sonnet. A 'veil' is a covering that hides what is beneath it; a 'blot' is a stain or moral blemish. The poet says the youth's beauty acts like a veil that transforms every fault into something the world reads as attractive. Because people can only see the surface, everything about the youth appears fair — the beauty literally converts the ugly to the beautiful for any observer.
historical The sonnet form - TLN 1330rhetorical device
“The hardest knife ill-us'd doth lose his edge”
The closing couplet ends on a proverb: even the finest knife, if used carelessly or for the wrong purpose, will go dull. 'His' is the Elizabethan possessive for 'its.' The lesson is that the youth's beauty — however exceptional — has limits. Misuse wears out even the best instruments. The couplet makes the warning of 'Take heed' specific: the privilege of beauty is not permanent.