following explains answers to Test Yourself: "That"
The data that that we gathered from
the experiment were conclusive.
In this sentence, the clause "that we
gathered from the experiment" is essential. Without this
clause, the sentence would be "The data were conclusive."
Although this short sentence is grammatically correct, the meaning
of the original is lost. Thus, because this clause is essential,
it requires the relative pronoun "that" and needs no
to Test Yourself: Sentences1-4