Paper submitted but not published strictly can not be cited as there are all chances that it may be rejected.While paper accepted by publisher and not published can always be cited as you have an evidence in writing that it has been accepted.
In principle you can cite other, submitted work in a research paper. Just give the authors, paper title, and either "Submitted." or "Submitted to [venue]." in the reference list.
However, both as a reviewer and reader, I usually find this disappointing. I already came across several cases where I wasn't able to find the cited paper even years after publication of the paper with the citation. It is well possible that the cited paper is rejected, and maybe someone just doesn't follow up to really get it published. As a better alternative, check whether you can put a preprint version of the paper you want to cite online (e.g. on arxiv), and just cite that.
Paper submitted but not published strictly can not be cited as there are all chances that it may be rejected.While paper accepted by publisher and not published can always be cited as you have an evidence in writing that it has been accepted.