Special Uses of "it" (Dummy Pronoun)

Pronoun "it" is usually used when you refer to something, for example: It is my home. So, when you say, "It is my home", you can show me which house you refer as "it". However, there are cases when you use "it" when you are not actually refer to anything, you can not show the thing you refer as "it".  This is called "Dummy Pronoun". There are three major non-referential uses of the pronoun "it" (as a "dummy pronoun").

a. empty subject
Empty "it" occurs when there are no participants to fill the subject slot. Usually when you refer to weather condition, time, or distance, for example:
- it is raining
- it is  five o'clock
- it is a long way from Quebec to Florida.

You can also find "it" as empty objects.

b. Anticipatory subject/object
"it" is inserted as a subject when a clause has been extraposed, for example:
- It would be fun to see a movie after work.
- It is amazing that you are finally married.

c.  Subject in cleft constructions
It is found in cleft constructions, when you want to put an emphasize on a particular element in the clause, for example:
- It is the key that you are looking for, isn't it? (means "you are looking for a key, not a pen, right?")
- It is tonight that I will leave. (I will leave tonight, not tomorrow.)

Sentence vs Proposition

Sentence : a grammatically complete string of words expressing a complete thought (at least contains a verb).

I saw a book on the table is a sentence.

Book on the table is not a sentence.

Proposition :Part of the meaning of the utterance of by declarative sentence that describes some state of affairs.

Example:

Take a look at this picture. You can describe this scene by:

A cat eats a mouse.
A mouse is eaten by a cat.

Those two different sentences belong to the same proposition. All those utterances can be analyzed as consisting of a predicate naming an event or state and on or arguments naming referents that participate in that event or state.

- The activity is eat

- The agent is cat

- The patient is mouse

Propositions, unlike sentences, cannot be said to belong to any particular language. Sentences in different languages correspond to the same proposition, if the two sentences are perfect translations of each other.

Look at this sentences:

I love you (English)

Aishiteru (Japanese)

Wo ai ni (Chinese)

We call them as different sentences and language, but they refer to the same proposition: I love you.