how can I copy a task and its associated sub-tasks?
You aren't able to do this. You can right click on a project, and use the Export feature which will export all tasks under that project.
1) right click on tasks does not give me anything
2) Exporting is way too techie and counter-intuitive. I have never been able to make it work. I am not a programmer
Sorry for the misunderstanding. You need to actually head into the Project, top right corner, there is button, then choose Export. This then downloads to a TXT file, which you can then follow the same steps and import.