Introducing a clause that is the object of a verb, especially a reporting verb or verb expressing belief, knowledge, perception, etc.
Introducing a clause that is the subject of a verb, especially the 'be' verb or a verb expressing judgement, opinion, etc.
Introducing a clause that is the subject of a verb, especially the 'be' verb or a verb expressing judgement, opinion, etc.
Introducing a clause that complements an adjective or passive participle.
Introducing a clause that complements an adjective or passive participle.
Introducing a subordinate clause modifying an adverb.
Introducing a clause that describes the information content of a preceding reporting noun.
Introducing — especially, but not exclusively, with an antecedent like so or such — a subordinate clause expressing a result, consequence, or effect.
Introducing a subordinate clause that expresses an aim, purpose, or goal ("final"), and usually contains the auxiliaries may, might, or should: so, so that, in order that.
Introducing a premise or supposition for consideration: seeing as; inasmuch as; given that; as would appear from the fact that.
Introducing an exclamation expressing a desire or wish.
Introducing an exclamation expressing a strong emotion such as sadness or surprise.

The (thing, person, idea, etc) indicated or understood from context, especially if more remote physically, temporally or mentally than one designated as "this", or if expressing distinction.

The thing, person, idea, quality, event, action, or time indicated or understood from context, especially if more remote geographically, temporally or mentally than one designated as "this", or if expressing distinction.
The thing, person, idea, quality, event, action, or time indicated or understood from context, especially if more remote geographically, temporally or mentally than one designated as "this", or if expressing distinction.
The thing, person, idea, quality, event, action, or time indicated or understood from context, especially if more remote geographically, temporally or mentally than one designated as "this", or if expressing distinction.
In a relative clause, referring to a previously mentioned noun, as subject, direct object, indirect object, or object of a preposition; which, who.
(colloquial) Used in place of relative adverbs such as where or when; often omitted.
Clipping of that is; used to reinforce the preceding assertion or statement.

To a given extent or degree.
To a given extent or degree.
To a great extent or degree; very, particularly.
(informal) To such an extent; so.

Something being indicated that is there; one of those.

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"What is it that I still need to know?" the boy asked.