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The Main Difference
The main difference between the two is that 'it' is a third-person singular personal pronoun, and 'this' is a demonstrative determiner and pronoun. Because of this difference in grammatical categories, they can perform different roles in a sentence.
Other Differences
In addition to their grammatical categories, 'this' and 'it' also differ in some other ways:
Pronoun
'It' is used with non-human, and non-living things. 'This' can be used to refer to humans, animals and things.
This is my friend, Allison. (NOT It is my friend, Allison.)
Look at that dog. It’s chasing the cat. (NOT this is chasing the cat.)
Identification and Introduction
'It' is used to identify a person. 'This' is used to introduce a person.
It is my brother.
'It' identifies a person. For example, you get a phone call from your brother, and you want to identify the caller to your friends. You identify by using 'it'.
This is my brother.
'This' introduces a person. For example, imagine that you want to introduce your brother to somebody. You introduce him by using 'this'.
Dummy Subject
'It' can be used as a dummy subject (for example, to talk about time, weather etc.) 'This' cannot be a dummy subject.
It is 12:00. (NOT this is 12:00.)
It's raining. (NOT this is raining.)
Demonstrative
'This' demonstrates a noun (it shows the nearness of an object to our eyes), 'it' replaces a noun already mentioned or known to the speaker.
This is a magnetometer.
'This is a magnetometer’ can only be used when we see a magnetometer in front of us, and we are showing or pointing to a specific magnetometer.
It is a magnetometer.
But, 'It is a magnetometer', can be used in many contexts, for example, when you want to Introduce the concept of a magnetometer to people who don't know what a magnetometer is. However, you must remember that with 'it', there's no indication of distance (both physical or chronological), and it doesn't matter if the magnetometer is within our sight or not.
Now let's compare these sentences:
What's this object on the shelf?
Option 1: This is a book.
Option 2: It's a book.
By choosing option 1, you are referring to a book that is near to you or that it is visible to you. But option 2 shows that it's a specific book, but not one which is necessarily within your sight.
Comments
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