bff7942 wrote:Do they have to introduce a clause (w/ a subject & a verb)??

no, no, and no; each of those 3 words has at least one grammatical function in which it does not precede a clause. i will demonstrate some of these functions in the following list (which is NOT exhaustive, nor intended to be exhaustive):

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THAT
...can be a pronoun:
despite popular misconceptions to the contrary, the annual rainfall of Miami is much greater than that of Seattle
...can be an adjective
that reason
that book

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WHETHER
...can introduce two parallel phrases/nouns/adjectives
people traveling to las vegas, whether on business or for pleasure, can take advantage of the city's spectacular nightlife any day of the week

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IF
...can be an approximate substitute for 'although'
confidence and arrogance are two distinct, if overlapping, traits
...can introduce participial phrases
if dropped from a height of more than six feet, your cell phone will break