For many reasons, lovers sometimes play games with one another. Such games may make the relationship more exciting. On other occasions, the games may help the lovers accept the faults in each other. Such a game is the topic in "Sonnet 138." In this particular case, the speaker and the Dark Lady tell deliberate lies to one another because the truth would affect the quality of their relationship. The truth would make them unhappy.
In regards to the structure of this sonnet, the division comes between the octet and sestet. In the octet the speaker explains what the lies are, but in the sestet he explains why he and the Dark Lady pretend that the lies are true.
In the first quatrain, the speaker presents a kind of paradox. He claims that he believes the Dark Lady even though he knows that she lies to him. How can a person believe something when that person knows it is a lie? It is a game. It is an act of imagination. The speaker and the Dark Lady play this game so that they can enjoy being with each other all the more. The word truth (in line 1) actually has two separate meanings here. First, the word refers to the truth accepted by the speaker when the Dark Lady lies to him. She tells the speaker that he is young ("some untutored youth") and innocent ("unlearned in … subtleties") even though he is old and experienced. The speaker accepts the truth of the statement even though he is painfully aware that he is