The word "that" is used in various ways in sentences, and these different functions may cause some confusion. The word "that" is used as a relative pronoun, a demonstrative pronoun, a demonstrative adjective, and a part of the word pair "so…that" used to show cause and effect.
The word "that" is a relative pronoun used to introduce relative clauses (also known as adjective clauses). Relative clauses describe nouns and pronouns. The common relative pronouns are who, which, and that. The word "that" is used to introduce an essential clause that is needed to completely identify the noun, especially if there may be confusion about which person or thing is being discussed in the sentence. Here are some sentences using the word "that" as a relative pronoun:
1. The truck that Mary bought was very practical. (There may be several trucks being discussed. The clause "that Mary bought" tells us which truck.)
2. The door that is in the back of the house needs a new lock. (There are several doors in the house. The door in the back is the one being discussed.)
3. The path that he took was the shortest one. (There are several paths, but he only took one of them, and it was the shortest of all the paths.)
Another word frequently used in relative clauses in the word "which." This word, unlike the word "that," is used to introduce material that is not essential to the meaning of the sentence. In other words, it introduces additional information that could be deleted, if necessary. Here are some sentences showing the difference between "which" and "that":
4. The vase that Margie broke was valuable. (Margie has a lot of valuable vases, but she only broke one of them.)
5. The blue vase, which Margie broke, was given to her by her grandfather. (The vase is completely identified; we don't need to know that she broke it.)
6. We visited the beach that Jill talked about. (There are lots of beaches, but Jill identified a specific one.)
7. We visited the beach, which has white sand, where Jill runs her dog. (We know which beach Jill visits with her dog. The fact that it has white sand is irrelevant.)
When the word "that" is used to modify another word, it is called a demonstrative adjective. When used this way, the word "that" specifies some specific choice the speaker is referring to. Here are examples:
8. Who was that guy? (The speaker is referring to a specific person in the situation being discussed.)
9. Please hand me that book. (The speaker is probably pointing to a specific book.)
When the word "that" is used alone without modifying another word, it is called a demonstrative pronoun. (Others are this, these, and those.) Often the antecedent of a demonstrative pronoun—the thing it refers to—is not clear. If the antecedent could be several different things, the sentence(s) should be rewritten to clarify the meaning. For example:
10. Comets usually pass by the earth, but meteors and asteroids sometimes hit the earth, causing damage. This interests astronomers.
The pronoun "this" in the second sentence above could refer to several things in the first sentence. The second sentence can be rewritten as follows:
11. This difference interests astronomers.
Here are some other examples of demonstrative pronouns:
12. That is a beautiful tree.
13. She criticized me for no reason. I didn't know what to say to that.
14. That was a close call!
15. This is mine and that is yours.
The word "that" is used in the word pair "so…that" to show a cause and effect relationship. In this case, the word "so" is attached to the phrase that is the cause of the action, and the word "that" is attached to the phrase that is the result. Here are some examples:
16. Brad drove so fast that he got a speeding ticket.
17. We had to rent a bicycle so many times last summer that we finally decided to buy one.
Note that both of the words in "so…that" must be used. It is improper to use the word "so" by itself as an adverb:
18. Brad drove so fast on his way over here. (This use of "so" by itself is incorrect!)
Here is one way to make the above sentence correct:
19. Brad drove extremely fast on his way over here. (Use another adverb to replace "so.")