This, That, These, Those 
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Demonstratives are used to point out a particular item. They  are as follows: 
This (indicates something close to the  speaker)
That (indicates something away from the speaker) 
These (indicates some things close to the  speaker)
Those (indicates some things away from the speaker) 
Example: 
A: What is 
this? (pointing to something held in the speaker's  hand or near the speaker) 
B: A pencil. 
A: What is 
that? (pointing to something at a distance from the  speaker) 
B: A tree. 
Remember that demonstratives are used in reference to the 
speaker.  (What is near
"you" may not be near "me" and vice versa.) 
A: What's that you're holding?
B: This is an egg. Here, catch!
A:  Oops.
B: That's a mess. A: What are these (holding up a pair of slippers)
B: Those are slippers.  
Be sure to use 
this/that with singular and non-count nouns and  
these/those with
count nouns. 
 | This orange is sour. | These oranges are sweet. | 
 | That truck is full. | Those trucks are empty. | 
 | This milk is fresh. |  | 
Sometimes  demonstratives can be used as pronouns (to refer to a particular noun.)   
 | Give me that! | Whose are these? | 
 | Give me that (spoon). | Whose are these (socks)? | 
This and 
that can also be used with  
one. However, 
these and 
those are  more
commonly used alone.  
 | I want this one. | I want these. (not these ones) | 
 | I'll take that one. | I'll take those. | 
On the telephone,  
this and 
that are used differently in British and  American English to
identify callers. Example: 
 | Receiver: | Hello. | 
 | Caller: | I was wondering if you would be interested in buying . . . | 
 | Receiver: | Who is this? (American) | 
 |  | Who is that? (British) | 
 |  | (Both expressions are used to ask the caller to identify  himself/herself.) | 
 | Caller: | This is Mr. Fuller from the Acme Brush Company. | 
 |  | (Same for both British and American English) | 
 | Caller: | Hi Jack. Long time no see! | 
 | Receiver: | Is this Bob? What a surprise! (American) | 
 |  | Is that Bob? What a surprise!  (British) |