Yet作adv,作未发生讲,或不该发生的意思 In writing or more formal speech, yet can come after 'not' : We do not yet have a solution to this problem.
Yet作conj. used to introduce a fact, situation, or quality that is surprising after what you have just said Kelly was a convicted criminal, yet many people admired him. She does not speak our language and yet she seems to understand what we say. a story that is strange yet true an inexpensive yet effective solution to our problem