“I do believe her though I know she lies,”: This line prompts the reader to question why Shakespeare would admittingly believe a lie. He could believe the woman he loves because a lie is easier to swallow than a harsh truth or possibly he believes her because their love is so strong that it can overcome any lies.
“That she might think me some untutor’d youth,”: This line is similar to the common phrase today “I wasn’t born yesterday,” and means that the woman he loves tries to fool him or lie to him but anyone would be able to