Can I Delete a Subject after I have started a Pattern?

Delete a Subject in Options

If you have started any work in Options, then: no, it is best to not delete a Subject (on the Subjects Screen).  There may be a pattern or choice or rule (etc) that contains it.

Instead, just leave it in the list (and just ignore it - it does no harm).

But if all you have done so far is enter (or import) your Subject List, then: yes, you can delete or edit any of those Subjects.