“It” can act as a regular pronoun (i.e., it refers to a noun) and can also act as a placeholder pronoun.

What do we mean by the term “placeholder pronoun”?

The term refers to a pronoun (“It”) that doesn’t refer to a noun and just holds a place in a sentence. For example:

It is nice that you are wearing jeans.

Is “it” referrring to anything in the above sentence?

No. “It” is just holding the place of a noun – in the above sentence, “it” is holding the place of the subject.

Let’s look at a couple of more sentences:

1. It is safe for you to go outside.
2. I want to make it clear that I was not a part of this crime.

In both the sentences above, “it” acts as a placeholder pronoun. “it” doesn’t refer to any noun. “It” holds the place of the subject in the first sentence and holds the place of the object in the second sentence.

I have seen that the placeholder “It” is commonly used when infinitives or that-clauses are the subject (or the object) of a clause.

For example:

It is nice that you are wearing jeans.

What do you think is nice per the above sentence?

Answer: That you are wearing jeans.

Right?

Thus, meaning-wise, the that-clause is the subject. The above sentence is equivalent to the following sentence:

That you are wearing jeans is nice.

Both these sentences mean the same and are grammatically correct. However, the first sentence with the placeholder “it” is more common nowadays.

The sentence “It is nice that you are wearing jeans” is an example of a sentence in which “it” acts as a placeholder and pushes the that-clause to the latter part of the sentence.

The same idea applies to infinitives.

It is safe for you to go outside.

What do you think is safe per the above sentence?

Answer: To go outside.

Right?

Thus, meaning-wise, the infinitive is the subject. Rather, the above sentence is equivalent to the following sentence:

To go outside is safe for you.

Both these sentences mean the same and are grammatically correct. However, the first sentence with the placeholder “it” is more common nowadays.

The sentence “It is safe for you to go outside” is an example of a sentence in which “it” acts as a placeholder and pushes the infinitive to the latter part of the sentence.

Similarly, in the below sentence, the placeholder “it” pushes the that-clause (which is meaning-wise the object of “make”) to the latter part of the sentence.

I want to make it clear that I was not a part of this crime.

Here are a few more sentences with the placeholder “it” and their equivalent sentences without the placeholder “it”.

1. It is very difficult to tell the male and female apart in blue-winged warblers.
   Equivalent Sentence: To tell the male and female apart in blue-winged warblers is very difficult.
2. It is perhaps surprising that the government decided to pass the bill.
   Equivalent Sentence: That the government decided to pass the bill is perhaps surprising.
3. It is not decided yet whether I’ll visit the US in November.
   Equivalent Sentence: Whether I’ll visit the US in November is not decided yet.

Are the above the only ways in which the placeholder “It” is used? 

I don’t think so. There are other ways too. However, the above ways are some common ways in which the placeholder “It” is used.

“One of them is blind” is an Independent Clause (IC) while “one of whom is blind” is a Dependent Clause (DC).

Why is the first an IC while the second is a DC?

Because ‘them’ is an object form of ‘they’ and ‘whom’ is an object form of ‘who’.

Identify whether the following clauses are independent or dependent.

1. They are blind
2. Who are blind

The first one is an IC while the second is a DC. 

Thus, their corresponding object forms also lead to IC and DC respectively.

The following are ICs:

1. They are blind
2. One of them is blind
3. Some of them are blind
4. Many of them are blind

The following are DCs:

1. Who are blind
2. One of whom is blind
3. Some of whom are blind
4. Many of whom are blind

ICs cannot be modifiers; DCs can be modifiers. Thus, if you need to modify a noun, you can use the “one of whom” construction to create a clause modifier.

For example:

My friends, all of whom are nice, will visit me today. (Correct. “all of whom are nice” is a DC that modifies “my friends”)

My friends, all of them are nice, will visit me today. (Incorrect. “all of them are nice” is an IC and thus cannot modify “my friends”; Grammar doesn’t allow an IC within double commas.)

No.

Since “their” cannot act as a noun. 

“their” is a possessive form of “they” and thus can act only as an adjective.

Can “his” act as a noun?

No. “his” can act only as an adjective just like “their”.

Similarly, “whose” can act only as an adjective and not as a noun.

“That” can refer to plural nouns ONLY when “that clause” acts as a modifier to the plural noun. For example, the following sentence is correct.

The apples that are rotten need to be thrown out.

In the above sentence, “that” refers to “apples” and is correctly used since “that” can refer to plural nouns when the “that clause” is modifying the plural noun.

However, when “that” is used not as a modifier but as a regular pronoun, “that” cannot refer to a plural noun; in such cases, “that” can refer to only singular nouns; “those” needs to be used to refer to plural nouns in such cases.

1. My nose is longer than that of Ram. (Correct. “that” refers to a singular “nose”)
2. My legs are longer than that of Ram. (Incorrect. “that” refers to plural “legs” – this is not allowed grammatically)
3. My legs are longer than those of Ram. (Correct. “those” refers to a plural “legs”)
4. My nose is longer than those of Ram. (Incorrect. “those” refers to singular “nose” – this is not allowed grammatically)

Yes, “they” can refer to animals and things too. For example, the following sentences are correct:

1. While apples are good for health, they are very expensive. (“they” refers to ‘apples’ – this is correct usage)
2. Since dogs have emotions, they should be treated with respect. (“they” refers to ‘dogs’ – this is correct usage)
3. While many people condemn the killings, they don’t dare to support the cause publicly. (“they” refers to ‘people’ – this is correct usage)

Yes, “whose” can refer to animals and things too. For example, the following sentences are correct:

1. People whose wisdom grows with time remain happy. (“whose” refers to ‘people’ – this is correct usage)
2. Dogs whose tails are shorter than 6 inches are more violent than other dogs. (“whose” refers to ‘dogs’ – this is correct usage)
3. The computers whose keyboards are poorly designed do not sell well. (“whose” refers to ‘computers’ – this is correct usage)

No. “whose” can refer to and modify both singular and plural nouns. For example, both below sentences are correct.

1. A computer whose keyboard is poorly designed does not sell easily. (“whose” refers to ‘computer’ – this is correct usage)
2. The computers whose keyboards are poorly designed do not sell well. (“whose” refers to ‘computers’ – this is correct usage)

No. “they”, “it”, and other pronouns such as “he” and “she” can refer to even faraway nouns, including nouns in another independent clause. e-GMAT has debunked common myths around pronouns in this excellent article.

Depends.

First of all, in this discussion, I want to restrict myself to the use of “that” in which “that clause” acts as modifier. (“that” can act as a pronoun without being a part of a “that clause” – its usage is different in this case)

The following pronouns can form clauses that act as modifiers to nouns:

1. that
2. which
3. who
4. whom
5. whose

The rule for every noun modifier is the same: a noun modifier can modifier the preceding noun or jump over other noun modifiers. A noun-modifier CANNOT jump over a verb or a modifier that is not a noun modifier.

Since all of the pronouns, when they modify a noun, refer to the same noun, the rule for their reference is the same as above: they can refer to the preceding noun or jump over other noun modifiers. They cannot refer to nouns that are separated from these pronouns with a verb or a modifier that is not a noun modifier.

Now, coming to the question asked:

Can “which” and “that” refer to nouns that are far?

These pronouns, when acting as modifiers, can refer to faraway nouns as long as they are separated only by noun modifiers from the noun they want to refer to. If they are separated from the noun with a verb or some verb modifier, they cannot refer to that noun.

Yes.

Only relative pronouns such as the following make the clauses dependent. Regular pronouns such as given below don’t make the clauses dependent.

Relative Pronouns: who, whom, whose, where, when, why, that, which and how.

Regular pronouns: 

1. He, his, him
2. She, her
3. You, your, yours
4. I, me, mine
5. It, its
6. They, them, their
7. We, us, our
8. That (can be used as a regular pronoun also)

“their” and “whose” both are possessive pronouns – they act as adjectives and do not act as nouns in a sentence. Please refer to the answer to the question “Can their act as a subject or an object in a sentence?” for more details.

“their” is a possesive form of “they” and “whose” is a possessive form of “who”.

Since a clause starting with “they” is an Independent Clause (IC) and a clause starting with “who” is a Dependent Clause (DC), a clause starting with “their” is an IC and a clause starting with “whose” is a DC. Reading the answer to the question above “Is there any difference between ‘one of them is blind’ and ‘one of whom is blind’?” will be helpful.