My husband was married before and he did petition for his wife a few years ago, she got her green card and he doesn't know about her anymore..we are not sure what to write under D-2 ( form I-130), should we leave it blank ? what to write especially under result ?
The answer is "yes." Result: approved and write the approximate date.
The question you're referring to asks "Have you ever before filed a petition for this or any other alien?" Based on your facts, you seem to already know the answer to this question.
The above information does not constitute legal advice, and is intended only as general information. You should speak with an attorney to discuss your individual case, as not all facts are known at this time.
If your husband has petitioned for someone before he must mark yes. It is important that you answer all questions honestly on any application.