It seems that in informal speech "that" can be used to replace any "who" or "which", which is why I suggest learners use it exclusively(in informal speech/writing), or remove it completely where they can so they won't make a mistake.
e.g. "There is a person who I think you should see."
"There is a person that I think you should see."
"There is a person I think you should see."
"This is a problem which will demand our attention."
"This is a problem that will demand our attention."
(alternatives include: This is a problem, and will demand our attention/ This problem will demand our attention)
"That" replaces "them" when we are dealing with the singular and not the plural.
e.g. "There are many problems, but I don't have time to talk about them right now.
"There is one problem, but I don't want to talk about that right now."
"There is a problem over there, could you go see to that?"
"There are a few problems, would you mind seeing to them?"
At least, that's the question I think you are asking.
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