As a demonstrative pronoun (pl. those), that usually points out, or refers to, a person or thing previously mentioned, or supposed to be understood. that, as a demonstrative, may precede the noun to which it refers; as, that which he has said is true; those in the basket are good apples.
As an adjective, that has the same demonstrative force as the pronoun, but is followed by a noun.
As a relative pronoun, that is equivalent to who or which, serving to point out, and make definite, a person or thing spoken of, or alluded to, before, and may be either singular or plural.
As a conjunction, that retains much of its force as a demonstrative pronoun.
To introduce a clause employed as the object of the preceding verb, or as the subject or predicate nominative of a verb.
To introduce, a reason or cause;
To introduce a purpose;
To introduce a consequence, result, or effect;
In an elliptical sentence to introduce a dependent sentence expressing a wish, or a cause of surprise, indignation, or the like.
As adverb: to such a degree; so; as, he was that frightened he could say nothing.