It is quite common to present talks or posters on the basis of previously published papers. However, care must be taken when contributing to the proceedings of the conference so as not to infringe the copyright of the journal's publisher.
No, a journal article that has been published cannot be submitted to a conference. For instance, authors are only permitted to submit original material that is either undergoing review for another peer-reviewed publication or has not been published before, according to IEEE regulation. This implies that authors should not submit material to a conference that has already been published in a journal; instead, they should acknowledge any past publications when submitting an article.
Mohammad Hossein Khajehpour yes you can, you just have to be careful not to publish your work in the conference proceedings or in a derivative paper. Regards!
yes, absolutely. A paper is a living organism, it may benefit from the new audience, feedback, question. Your copy right, as an author, is absolute. The journals live on you, not you on them. Also, I found in many places the papers published in multiple formats, as part of journals or conference-proceedings, and also as parts in a book by the same author. These are just different cloutings. The key paper is in your head as an author. The same, more or less, applies to collective authorship.