A表、B表是怎么回事？
-If your priority date is earlier than the cut-off date in A Form, then that means the visa number is available to your case. Thus, you can file I-485 or your I-485 may be approved (if you have a pending I-485); If your priority date is earlier than the cut-off date in B Form, then National Visa Center may ask you to provide the documents and you can file DS 260 to apply for the immigrant visa outside US; sometimes you may also can file I-485 within US using B Form if you are present in US.
Whether you can file I-485 based B Form, you need to check this website each month:
Adjustment of Status Filing Charts from the Visa Bulletin | USCIS
如果B表 current, A表还没有，可以做什么？
-See above link to see whether you can file I-485 if you are present in US.
