I have deleted all my tags on purpose but I still have 2 tags with 4 related tasks except I can't find those tasks. How can I delete that (tag and/or tasks)?
ElynnWQ, your suggestion will probably work, but tasks that were completed more than a week ago won't show up in any of the pre-defined Views, even if you turn off all filters.
In that case, you would need to create a Search that includes completed tasks.
Well, that depends on what exactly you want to display ... ;)
If you haven't created a Search before, it's a bit difficult to tell you how to do it. Best to read the link that I provided in my first post.
If you already have some experience with Searches, then create a new Search and delete "Checked off = No" (by clicking on the X at the far right), and add "Tag contains" whatever Tag you're looking for.