"That" instead of "those" with plural nouns

Question

At the first glance, that is used with singular nouns, and those with plural.

However, sometimes I see that that is used with plural nouns as well.

So, both of these examples seems to be correct:

The obvious one:

The car that belongs to me needs to be repaired.

The not-so-obvious one:

The cars that belong to me need to be repaired. (That, not those.)

External references:

1. https://www.englishforums.com/English/ThatVsThose/pxnwl/post.htm

2. A quote from LibreOffice Writer user guide:

You can easily check which properties are specific to a style by looking at the Contains section of the Organizer page. If you want to reset the properties of a child style to that of the parent style, click the Standard button located at the bottom of each Paragraph and Character style dialog.

3. https://dictionary.cambridge.org/grammar/british-grammar/relative-pronouns

How can I understand should I use "that" or "those" in such cases?

Answer 1

In these sentences, that is a conjunction. It introduces a relative clause. That only needs to change to those when it's an adjective modifying a plural noun or a pronoun referring to a plural antecedent.

The car that belongs to me needs to be repaired.
The cars that belong to me need to be repaired.

The sentence from the LibreOffice Writer user's guide is an error. It should say:

If you want to reset the properties of a child style to those of the parent style, click the Standard button located at the bottom of each Paragraph and Character style dialog.

By the way, that can also be an adverb. When it's an adverb, it also doesn't agree in number with anything else. For example:

My car isn't that fast.
My cars aren't that fast.