Half right. Try again.

Susie's theory <that studying is helpful> reinforced that of John.

Susie's theory is helpful? Maybe, but that's not what this sentence says. What does Susie's theory DO in this sentence?

Great!

Most of the time, if you see "that" after a noun, then you're looking at a noun modifier (and the "that" modifier is modifying the noun before it, though it may also be modifying not that noun but the main noun before it - eg, The box of nails that I opened. I opened the box, not the nails.).

The "that" modifier is actually using the word "that" as a relative pronoun.

Then, separately, "that" can also be used as a necessary extension in a core sentence. Most of the time, the word "that" (in this case) will follow a verb.

She believes THAT studying is important.