I know for a fact that for some verbs like 'say', 'indicate', 'know', 'claim', the deletion of 'that' is usually optional, though it's safer to include it. However, does the same apply when 'that' is used to linked a subject to a independent clause?
To clarify with an example:
The worst thing is you don't even know who did it.
The worst thing is THAT you don't even know who did it.
Which one is grammatically correct, if not both?
Note: I am having trouble finding something to refer to that specifically discusses the omitting of 'that' after verb-to-be. Providing me with a link would be greatly appreciated.
Many thanks in advance.