This, that, these, those are known as Demonstrative Pronouns and their purpose is to point at the position of a person or object in time and space. They can be either Singular or Plural.
· Please note that the Demonstrative Words can be used either as Pronouns or Adjectives depending on whether they are being followed by the verb or the Noun in a sentence.
1. Demonstrative Adjectives never stand alone or play the role of Noun or Noun Phrase in a sentence. They just help to qualify the Nouns.
e.g.: These apples taste rotten.
These pencils draw and those pens write. (Example of Demonstrative Adjectives qualifying the Nouns that follow)
2. The Demonstratives function as Pronouns when they point out to the position of an Object or Person, near/far in space.
e.g.: These draw but those write. (Pointing out where the pencils and pens that draw and write are located, respectively)
A. They take the place of Noun or Noun Phrase and stand alone.
e.g.: The lunch you are cooking smells delicious. (Use of Noun Phrase as Subject)
This smells delicious. (Demonstrative Pronoun as Subject)
The cockroaches under your sink are totally gross.(Use of Noun Phrase as Subject)
Those are totally gross.(Demonstrative Pronoun as Subject)
B. They are used to replace specific people or things that have been previously mentioned or understood in the larger context.
C. A Demonstrative Pronoun replacing a subject can either function as a Subject or an Object.
E.g.: This is Mary.
These are the grapes.
D. Like any other Subject, they must agree with the verb that follows :
e.g.: This is extremely smelly.
You can smell that from a mile.
These apples seem rotten.
Those apples look rotten.
E. At time words like Such, More and neither are also used as Demonstrative Pronouns replacing the Noun phrase in a sentence.
e.g.: He will allow none.
Neither will work over here.
Such is the human race that it not an ounce of pity for its own ilk.
F. In order for Demonstrative Pronouns to work perfectly, either the Antecedent must make it amply clear or the listener needs to be in close proximity to the Speaker to follow his directions.
It is for this obvious reason that Demonstrative Pronouns are more commonly used in the Conversational format.
However, it can be used in the written English too if the context makes clear the Noun to which the Demonstrative Pronoun refers.
e.g.: Raindrops, petrichor, gusts of wind, cotton ball like clouds, these are a few of my favorite things.
G. Demonstrative Pronouns always identify Nouns, no matter how abstract or vague they be.
e.g.: I can’t believe this.(Vague but definitely exists)
H. Demonstrative Pronouns are usually used to pin-point animals, places or things but people can also fall in this bracket.
e.g.: This sounds like Md. Rafi.
Some more Examples:
i. This was my family heirloom.
ii. Those are nice but look uncomfortable.
iii. Neither of the horses can be taken.
iv. Such was her dexterity that everyone was impressed.
v. None of your conjectures are valid.