This and It are two words that are definitely used with some difference between them. The word ‘this’ is used as a demonstrative pronoun whereas the word ‘it’ is used as a personal pronoun.
The word ‘this’ is used to demonstrate a noun that has already been uttered or used in a sentence. Look at the sentences
1. This looks a good one
2. I am ready to buy this.
In both the sentences mentioned above the word ‘this’ is used as a demonstrative pronoun. In the first sentence the word ‘this’ may refer to a flower, a book or any other object that was already used once in a sentence.
In the second sentence the word ‘this’ may refer to a toy, a book or an ornament that was already mentioned in a sentence uttered before the present sentence.
The same sentences mentioned can be uttered completely as follows:
1. This flower looks the good one.
2. I am ready to buy this book.
Here again the word ‘this’ is used as a demonstrative pronoun.
The word ‘it’ is used mainly as a personal pronoun in the third person. The word ‘it’ is used in the same way as we use ‘he’ and ‘she’ as in the sentences
1. He looked at it with wonder.
2. He gesticulated at it to keep quiet.
Here the word ‘it’ is used in the third person as a personal pronoun referring to an animal such as a dog. It is interesting to note that the word ‘it’ is used in exclamation too as in the sentence, “How beautiful it is!”
It is equally interesting to note that the word ‘this’ is used in affirmative sentences too as in the “This is the answer”. The words ‘it’ and ‘this’ are to be used with great care and precision.
This was the link given by Usha
