pron.1. (used to indicate a person, thing, idea, state, event, time, remark, etc., as pointed out or present, mentioned before, supposed to be understood, or by way of emphasis): That is her mother. After that we saw each other.2. (used to indicate one of two or more persons, things, etc., already mentioned, referring to the one more remote in place, time, or thought; opposed to this): This is my sister and that's my cousin.3. (used to indicate one of two or more persons, things, etc., already mentioned, implying a contrast or contradistinction; opposed to this): This suit fits better than that.4. (used as the subject or object of a relative clause, esp. one defining or restricting the antecedent, sometimes replaceable by who, whom, or which): the horse that he bought.5. (used as the object of a preposition, with the preposition standing at the end of a relative clause): the farm that I spoke of.6. (used in various special or elliptical constructions): fool that he is.7. at that,a. in spite of something; nevertheless: Although perhaps too elaborate, it seemed like a good plan at that.b. in addition; besides: It was a long wait, and an exasperating one at that.8. that is, (by way of explanation, clarification, or an example); more accurately: I read the book, that is, I read most of it. Also, that is to say.9. that's that, Informal. there is no more to be said or done; that is finished: I'm not going, and that's that!10. with that, following that; thereupon: With that, he turned on his heel and fled.adj.11. (used to indicate a person, place, thing, or degree as indicated, mentioned before, present, or as well-known or characteristic): That woman is her mother. Those little mannerisms of hers make me sick.12. (used to indicate the more remote in time, place, or thought of two persons, things, etc., already mentioned; opposed to this): This room is his and that one is mine.13. (used to imply mere contradistinction; opposed to this): not this house, but that one.14. that way, Informal. in love or very fond of (usually fol. by about or for): The star and the director are that way. I'm that way about coffee.adv.15. (used with adjectives and adverbs of quantity or extent) to the extent or degree indicated: that much; The fish was that big.16. to a great extent or degree; very: It's not that important.17. Dial. (used to modify an adjective or another adverb) to such an extent: He was that weak he could hardly stand.conj.18. (used to introduce a subordinate clause as the subject or object of the principal verb or as the necessary complement to a statement made, or a clause expressing cause or reason, purpose or aim, result or consequence, etc.): I'm sure that you'll like it. That he will come is certain. Hold it up so that everyone can see it.19. (used elliptically to introduce an exclamation expressing desire, a wish, surprise, indignation, or other strong feeling): Oh, that I had never been born![bef. 900; ME; OE thaet (pronoun, adj., adv. and conj.), orig., neut. of se the; c. D dat, G das(s), ON that, Gk tó, Skt tad]Usage. 4. When THAT introduces a relative clause, the clause is usually restrictive; that is, essential to the complete meaning of the sentence because it restricts or specifies the noun or pronoun it modifies. In the sentence The keys that I lost last month have been found, it is clear that keys referred to are a particular set. Without the THAT clause, the sentence The keys have been found would be vague and probably puzzling. THAT is used to refer to animate and inanimate nouns and thus can substitute in most uses for who(m) and which: Many of the workers that (or who) built the pyramids died while working. The negotiator made an offer that (or which) was very attractive to the union.Experienced writers choose among these forms not only on the basis of grammar and the kind of noun referred to but also on the basis of sound of the sentence and their own personal preference.The relative pronoun THAT is sometimes omitted. Its omission as a subject is usually considered nonstandard, but the construction is heard occasionally even from educated speakers: A fellow (that) lives near here takes people rafting. Most often it is as an object that the relative pronoun is omitted. The omission almost always occurs when the dependent clause begins with a personal pronoun or a proper name: The mechanic (that) we take our car to is very competent. The films (that) Chaplin made have become classics. The omission of the relative pronoun as in the two preceding examples is standard in all varieties of speech and writing.18. The conjunction THAT, which introduces a noun clause, is, like the relative pronoun THAT, sometimes omitted, often after verbs of thinking, saying, believing, etc.: She said (that) they would come in separate cars. He dismissed the idea (that) he was being followed. As with the omission of the relative pronoun, the omission of the conjunction almost always occurs when the dependent clause begins with a personal pronoun or with a proper name. This omission of the conjunction THAT occurs most frequently in informal speech and writing, but it is a stylistic option often chosen in more formal speech and writing.
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