Since the version of the word "that" might matter to the correct answer, I'll provide the actual title in question:
Title: Existing Solutions that Didn't Work
If someone could explain the why of the answer, I would be appreciative.
EDIT: It has been suggested that this is a duplicate, and that may be, in a sense. However, the after reading the suggested prior question, I'm no closer to my answer. I think it may have to do with my use of the word "that." Perhaps it is incorrect?
Using these definitions of the word "that:" I'm having trouble figuring out which definition of that matches my title. Am I even using it correctly? Perhaps I should use which instead:
Possible Title: Existing Solutions which Didn't Work
If my original usage is incorrect, and I should use which, then the answer is simple since which is always a pronoun or adjective, and those are both capitalized in a title. However, I'm having difficulty understanding which part of speech that is in my original title.