if i have completed some project, could i complete it or those projects?
If you'd like to complete a project you would need to have a Premium account, with Premium you can Archive a project, all you need to do is press the "gear" icon beside the project name and hit Archive once Premium.
This will move all projects and tasks underneath that project to an Archive list. Will be as completed.
Why such a difficult process? I have a premium account and completing a task should be easier in my opinion.
To complete a task, all you have to do is to check it off. However, projects can't be completed as such, but you can archive them by right-clicking on a project and choosing "Archive".