that

Từ điển American Heritage Dictionary 4th

that (thăt, thət)pron. pl. those (thōz)
1.
a. Used to refer to the one designated, implied, mentioned, or understood:
What kind of soup is that?
b. Used to refer to the one, thing, or type specified as follows:
The relics found were those of an earlier time.
c. Used to refer to the event, action, or time just mentioned:
After that, he became a recluse.
2. Used to indicate the farther or less immediate one:
That is for sale; this is not.
3. Used to emphasize the idea of a previously expressed word or phrase:
He was fed up, and that to a great degree.
4. The one, kind, or thing; something:
She followed the calling of that which she loved.
5. those Used to indicate an unspecified number of people:
those who refused to join.
6. Used as a relative pronoun to introduce a clause, especially a restrictive clause:
the car that has the flat tire.
7.
a. In, on, by, or with which:
each summer that the concerts are performed.
b. According to what; insofar as:
He never knew her, that I know of.adj. pl. those
1. Being the one singled out, implied, or understood:
that place; those mountains.
2. Being the one further removed or less obvious:
That route is shorter than this one.adv.
1. To such an extent or degree:
Is your problem that complicated?
2. To a high degree; very:
didn't take what he said that seriously.conj.
1. Used to introduce a noun clause that is usually the subject or object of a verb or a predicate nominative: “That contemporary American English is exuberantly vigorous is undeniable” (William Arrowsmith).
2. Used to introduce a subordinate clause stating a result, wish, purpose, reason, or cause:
She hoped that he would arrive on time. He was saddened that she felt so little for him.
3.
a. Used to introduce an anticipated subordinate clause following the expletive it occurring as subject of the verb:
It is true that dental work is expensive.
b. Used to introduce a subordinate clause modifying an adverb or adverbial expression:
will go anywhere that they are welcome.
c. Used to introduce a subordinate clause that is joined to an adjective or noun as a complement:
was sure that she was right; the belief that rates will rise soon.
4. Used to introduce an elliptical exclamation of desire:
Oh, that I were rich!Idioms:at that
1. In addition; besides:
lived in one room, and a small room at that.
2. Regardless of what has been said or implied:
a long shot, but she just might win at that.that is
To explain more clearly; in other words:
on the first floor, that is, the floor at street level. [Middle English, from Old English thæt. See to- in Indo-European Roots.] 
Usage Note: The standard rule requires that that should be used only to introduce a restrictive (or defining) relative clause, which identifies the entity being talked about; in this use it should never be preceded by a comma. Thus, in the sentence The house that Jack built has been torn down, the clause that Jack built is a restrictive clause identifying the specific house that was torn down. Similarly, in I am looking for a book that is easy to read, the restrictive clause that is easy to read tells what kind of book is desired. A related rule stipulates that which should be used with nonrestrictive (or nondefining) clauses, which give additional information about an entity that has already been identified in the context; in this use, which is always preceded by a comma. Thus, we say The students in Chemistry 101 have been complaining about the textbook, which (not that) is hard to follow. The clause which is hard to follow is nonrestrictive in that it does not indicate which text is being complained about; even if the clause were omitted, we would know that the phrase the textbook refers to the text in Chemistry 101.·Some grammarians extend the rule and insist that, just as that should be used only in restrictive clauses, which should be used only in nonrestrictive clauses. Thus, they suggest that we should avoid sentences such as I need a book which will tell me all about city gardening, where the restrictive clause which will tell me all about city gardening indicates which sort of book is needed. But this extension of the rule is far from universally accepted, and the use of which with restrictive clauses is common. Furthermore, since that cannot be used with clauses introduced by a preposition (whether or not restrictive), which is used with both clauses when such a clause is joined by and or or to another that does not begin with a preposition, as in It is a philosophy in which the common man may find solace and which many have found reason to praise. Such constructions are often considered cumbersome, however, and it may be best to recast the sentence completely to avoid the problem.·That is often omitted in a relative clause when the subject of the clause is different from the word that the clause refers to. Thus, we may say either the book that I was reading or the book I was reading. In addition, that is commonly omitted before other kinds of subordinate clauses, as in I think we should try again where that would precede we. These constructions omitting that are entirely idiomatic, even in more formal contexts. See Usage Note at doubt. See Usage Note at this. See Usage Note at whatever. See Usage Note at which. See Usage Note at who.