1. THIS/THAT – THESE/THOSE

We can use THIS and THAT as adjectives or pronouns. The plural form of these words are THESE and THOSE respectively.

A)  We can use THIS, THESE, THAT, and THOSE for referring to people/things/ events that have already been mentioned; e.g.

i) Everything is getting more expensive and this is something very tough for the poor.
ii) So, for all these reasons, my advice is to be very, very careful. (It’s understood that REASONS have already been mentioned.)
iii) How about natural gas? Is that an alternative?

B) We also use THAT or THOSE when we are referring to something for the  second  time in a sentence, using the same noun; e.g.

i) I know that what I say to a person is seldom what that person hears.
ii) My school teacher has suggested some books, at present I’m unable to get those books.

C) We use THAT (not THIS) to refer to ideas associated with another person; e.g.

i) She was terribly afraid of offending anyone. — That’s right.
ii) That’s a good point, he said in response to my question.

D) We can use THAT and THOSE as substitutes for THE ONE and THE ONES; e.g.

The most important information is the one given at the beginning of the document.
= The most important information is that given at the beginning of the document.

The rules being followed are the ones familiar to the participants.
= The rules being followed are those familiar to the participants.

NOTE-I: Similarly we can use THAT OF and THOSE OF instead of THE ONE OF and THE ONES OF; e.g.

i) The proton has a similar mass to that of a neutron.
ii) The emotions in this movie are those of laughter.

NOTE-II: THAT is used only for things, not for persons or animals, whereas we use THOSE for persons also; e.g.

INCORRECT: I’ve already met Sushma, that who works at the town hall.
CORRECT: I’ve already met Sushma, the one who works at the town hall.

There are coaching classes for all SSC exams. Those interested can take admission in any.

2. IT 

A) For things, animals, and small children whose sex we don’t know, we use IT; e.g.

i) He brought a tray and put it on the table.
ii) Look at that bird. It is so lovely!
iii) The noise went on for hours, then it suddenly stopped.
iv) Her new baby is tiny. It only weighs 2 kilos.

NOTE: We also use IT for people in sentences such as:

i) Who is at the door? – It is I.
ii) Is that Pravesh over there? — No, it’s Suraj.

B) We also use IT in expressions of time, day, date, distance, weather, temperature, light, etc.; e.g. 

i) It’s seven o’clock.
ii) It’s Sunday morning.
iii) It’s the fifth of December.
iv) It was a fine evening.
v) It’s getting dark.
vi) How far is it to Amritsar? — It is 500 kilometres.
vii) It’s full moon tonight.

C) We can also use IT to refer to a situation, fact, or experience; e.g.

i) I like it here.
ii) He was frightened, but tried not to show it.
iii He smokes in bed, though I don’t like it.
iv) He suggested flying, but I thought it would cost too much.

NOTE: But to express an opinion using an -ing form or to- infinitive after a verb such as LIKE, we don’t use IT in front of  neither the -ing form nor infinitive; e.g.

INCORRECT: I like it walking in the park.
CORRECT: I like walking in the park.

INCORRECT: I prefer it, to make my own food.
CORRECT: I prefer to make my own food.

D) We can use IT with an adjective or noun phrase to comment on a whole situation.  In that case we use a THAT-CLUSE after the adjective or noun phrase; e.g.

i) It is lucky that he didn’t break the glass.
ii) It’s a pity you can’t stay longer.

NOTE: After an adjective, we can sometimes use a WH-CLAUSE instead of a that-clause; e.g.

i) It’s funny how people change.
ii) It’s amazing what you can discover in the library.

3. IT/THERE as artificial subjects

A) Every sentence/clause must have a subject. Sometimes we need to use an artificial subject when there is no subject attached to the verb, and where the real subject is somewhere else in the sentence/clause.  IT and THERE are the two words that we use as artificial subjects in English; e.g.

i) It’s always good to keep all important documents safe in a file.

[The real subject here is to keep all important documents safe in a file. So this sentence can be re-written as ‘To keep all important documents safe in a file is always good.]

ii) There are two temples in our colony.

[The real subject here is two temples. So this sentence can be re-written as ‘Two temples are in our colony.]

NOTE-I: With adjectives We use IT (not THERE) as an artificial subject with adjectives; e.g.

i) It’s important to wear a helmet whenever you ride a bike.
2. It’s useful to write down your driving license number somewhere, in case you lose it.

NOTE-II: When something/someone exists or is present in a particular place or situation, we don’t use IT, we use THERE instead; e.g.

INCORRECT: It’s a lot of noise in the room today.
CORRECT: There’s a lot of noise in the room today.

INCORRECT: It’s a woman waiting outside who wants to talk to you.
CORRECT: There’s a woman waiting outside who wants to talk to you.

INCORRECT: It were no children playing in the park.
CORRECT: There were no children playing in the park.

B) IT can also be used with a plural noun; e.g.

It’s more teachers that we need now.

4. IT/THIS/THAT

We use IT to continue to refer to the topic already mentioned; e.g.

i) Our school library is very big. It has as many as 20,000 books. (IT refers back to OUR SCHOOL LIBRARY.)
ii) My father has just recently written a novel. It is his fifth novel.

NOTE-I: But when we give information about a topic for the first time we use THIS (not IT); e.g.

INCORRECT: It must be signed by all members and returned by tomorrow.
CORRECT: This must be signed by all members and returned by tomorrow.

NOTE-II (a): THIS can refer back to an entire idea, not just a single noun; e.g.

INCORRECT: It is going to rain today. It means we can’t have a picnic.
CORRECT: It is going to rain today. This means we can’t have a picnic.

INCORRECT: The price of gas has fallen. It is one reason why people are driving more.
CORRECT: The price of gas has fallen. This is one reason why people are driving more.

INCORRECT: Heavy rains throughout the week have led to severe shortages in some fruits. It has led to price rises in many markets.
CORRECT: Heavy rains throughout the week have led to severe shortages in some fruits. This has led to price rises in many markets.

NOTE-II (b): We use THAT (not THIS) to refer to ideas associated with another person; e.g.

i) She was terribly afraid of offending anyone. — That’s right.
ii) That’s a good point, he said in response to my question.
iii) The Principal apologised for the poor results this year and promised for better efforts from the teaching staff. That was a promise parents were very happy at.