Can we ever use "that" and "who" right next to each other?

Question

I’ve learned that we can use that to provide more information for abstract nouns, such as problem, belief, etc. I don’t quite understand what that means, though, so let me try it out.

For example, here I’ll “use that to provide more information about the word problem” by placing it right at the very front of a clause serving as the predicate complement after “The problem is” below:

1. The problem is that who will go.
2. The problem is who will go.

Can I use sentence #1 here in place of sentence #2? Does that sound ok that way? Would that mean that “that who will go” counts as a thing, or if not, that only “who will go” counts as a thing? What’s the difference between those two subordinate clauses?

More generally, does English syntax ever allow for using that and who right next to each other like I’ve just done above?

• If so, does this have any special name for it in grammar?

• If not, how does this rule that they’ve taught us that we can “use that to provide more information for abstract nouns like problem” play out in the real world? If my suggested example in sentence #2 with that who next to each other is wrong, what then would be a correct example of that which would clearly demonstrate this rule they’ve taught us about the word that?

Answer 1

[1] *The problem is that who will go.

[2] The problem is who will go.

[1] is ungrammatical. The subordinator "that" can introduce declarative content clauses but not subordinate interrogative clauses (like "who will go").

[2] is acceptable, though normally we find "The problem is: Who will go?"

Answer 2

Your example doesn't work as it stands. You would have to say:

The problem is - who will go?

where the question forms a separate clause, or:

The problem is that we don't know who will go.

Answer 3

As @BillJ pointed out, "that" needs to introduce a declarative clause, not an interrogative.

That can still have "who" immediately following "that" in a few cases, such as: "The problem is that who will go has not yet been decided."

In this case, we have a clause of the form "X as not yet been decided", which is a declaration. It just happens that in this case the noun "X" is "who will go".

Answer 4

"That" can be a pronoun ("I saw that"), a determiner ("I want that one"), or a subordinating conjunction ("It's important that you listen"). You seem to be asking about its use as a subordinating conjunction. Subordinating conjunctions begin subordinate clauses, and that clause functions as a single grammatical unit within the larger sentence.

For instance, in "It's important that you listen", "that you listen" is a subordinate clause, and functions as a noun phrase, and it is the subject complement of the main verb of the sentence, "is".

When you say "that who will go", the entire phrase "who will go" is acting as a noun phrase; the entire phrase refers to a person. So now "that" is not introducing a subordinate clause; a clause needs a verb, and while "will" is a verb, it is a verb within a noun phrase, and so the conjunction "that" doesn't "see" it as a verb. To have another subordinating conjunction, we need another verb. For instance, "The problem is that who will go hasn't been chosen". That is technically grammatically valid, but awkwardly/confusingly worded.