I haven't found any concise explanation of this.
How do I install mono's System.Windows.Forms on Ubuntu?
ubuntu;package management;software installation;mono
So you're looking for a package containing a file called System.Windows.Forms.dll. You can search:on your machine: apt-file search System.Windows.Forms.dll (the apt-file package must be installed)online: at packages.ubuntu.com.Both methods lead you to (as of Ubuntu 14.04):libmono-system-windows-forms4.0-cil and libmono-winforms2.0-cil.Install it with:sudo apt-get install libmono-system-windows-forms4.0-cil