In the English language, there are four words which can be used as demonstrative determiners, better known as demonstrative adjectives or demonstrative pronouns depending on their purpose in a sentence. The four most common demonstrative adjectives in English are this, that, these and those. This and that refer to singular nouns, while these and those refer to plural nouns.
What demonstrative adjectives do is designate a specific noun as opposed to a general noun prefaced with a, an or the: "I want THAT pair of pants," "THIS shirt is not clean," "THOSE shoes do not match," "THESE towels smell funny." The reader should understand exactly which item the speaker is referring to in each sentence. Sometimes it helps to form a mental image of the speaker physically pointing out, or demonstrating, the specific noun in question.
Another function performed by demonstrative adjectives is the establishment of distance from the speaker. The demonstrative adjectives this and these imply that the object is relatively close to the speaker. When a person writes "I want THIS box moved to another room," for example, the reader should get the impression the box is physically near the speaker. If a sentence reads "I want THESE things gone by the time I return," the reader should realize the objects are relatively close. The singular this and the plural these both indicate close proximity.
The demonstrative adjectives that and those, on the other hand, imply a certain amount of distance from the speaker. A sentence might read "THAT clock needs to be adjusted," or "THOSE clothes need to be folded and put away." A reader should be able to add the words over there when reading a sentence containing that or those. At one point in history, there were also two other demonstrative adjectives in common use; yon and yonder. Yon and yonder, which have largely fallen out of popular usage, implied an even greater physical distance between the speaker and the object(s): "The dog is sleeping in YON shed" or "The workers are fixing YONDER shed".
Whenever demonstrative adjectives are used by themselves in place of a proper noun, they are considered demonstrative pronouns. A sentence containing a demonstrative pronoun might read "THIS is the kind of place many people dream of visiting" or "THESE are the times that try men's souls." The speaker could have used this or these as demonstrative adjectives, as in "THESE times try men's souls." but the reader should understand the implied reference. The improper use of a demonstrative pronoun, however, can create confusion or ambiguity: "The man slipped on a banana peel and a policeman ran to help him. THIS made all of the children laugh." The demonstrative pronoun this could refer to either the man or the police officer's actions, which makes the sentence confusing to the reader. Demonstrative adjectives should only be used as demonstrative pronouns as long as the reference is clear and unambiguous.