“thou know'st I am forsworn”
'Forsworn' means guilty of breaking a solemn oath. In Elizabethan England an oath was sworn before God, making its violation not just a social failing but a spiritual crime. The poet admits he is forsworn by loving the Dark Lady; she is 'twice forsworn' because she broke her marriage vow to be with him, and then swore new love while bearing new hate.
historical The Fair Youth, the Dark Lady, and the Rival Poet“In act thy bed-vow broke”
'Bed-vow' is the vow of fidelity sworn at marriage — specifically the promise of sexual faithfulness made in the context of the marriage bed. 'In act' means in deed, not merely in intention: the Dark Lady has broken this oath by sleeping with the poet. This makes her affair with him adultery, not just romantic disloyalty.
“sworn thee fair; more perjur'd I, To swear against the truth so foul a lie”
'Sworn thee fair' carries two meanings at once: to have sworn an oath that she is fair (beautiful), and to have sworn in an honest or fair manner. Both senses are simultaneously false. The Dark Lady is dark-complexioned, not conventionally fair; and to call her fair required perjuring the evidence of his own sight. The couplet collapses aesthetic and moral judgment: 'foul' is the direct antonym of 'fair' in both senses — ugly and dishonest.
historical The blazon (and the anti-blazon)historical The Fair Youth, the Dark Lady, and the Rival Poet