How we cite our quotes: (Chapter.Page)
Quote #4
'[Mark] is very angry with me because of his evil counsellors; he is very wrong to believe them, he does not realize how they have deceived him. (2.50)
Tristan and Yseut continually insist that the barons are deliberately deceiving Mark. The irony, of course, is that Tristan insists this while pulling Mark's leg and insisting that he and Yseut are not lovers.
Quote #5
'Tristan, queen, listen to me carefully: to escape the shame and to cover up the wrong we shall have to think of some suitable falsehoods.' (12.100)
The "suitable falsehoods" that Ogrin advises Tristan and Yseut to tell Mark are that they have never been lovers and that Tristan only fled with Yseut to protect her. The fact that it's a religious man advising them to engage in this deception might be the story's way of saying this behavior on the part the two lovers is okay. This also suggests that Tristan and Yseut always did right to attempt to hide their adultery from Mark.
Quote #6
'Where are you from, leper?' asked the king. 'From Caerleon, the son of a Welshman.' 'How many years have you been living away from people?' 'For three years, sire. I am not lying. For as long as I was healthy I had a very noble lover, now because of her I have these bumps on my skin.' (15.132)
Here, Tristan pretends to be a leper in order to help Yseut with the oath she is about to swear. Like the oath she swore to Mark earlier, Yseut's deception depends on Mark's misunderstanding of someone's identity in order to work. Notice that much like Yseut's equivocal oaths, a lot of what Tristan says to Mark here is true: he has been living away from people for about three years, and it is because of his noble lover—Yseut—that he has bumps on his skin, since it was she who prompted him to don his disguise.