If we look at your examples from a strictly structural view, a wonderful pattern emerges.
Originally Posted by navi tasan
First, if we interpret 'It' as a pronoun meaning, say, 'That thing we saw', then sentence 1- works (given Tdol's suggested context):
1- It (That thing we saw) was a gun on the table.On the other hand, if we interpret 'It' as an expletive, the sentence reads awkward because the phrase 'on the table' is interpreted as part of the semantic subject (i.), but it is not. It is an adverbial complement (ii.), structurally separate from the noun it modifies, and as such requires a pronoun with adverbial properties, which is why 'There' is used (iv.):
I see a similar pattern in your second example:
i. It was a gun on the table
ii. A gun
was on the table.
is a gun on the table.
2. That was a gun on the table.
The pronoun 'That' refers to 'a gun', whereas the omitted pronoun 'there' refers to 'on the table'. In your example 2-, 'That' is short for 'That (there)' as evidenced by the symmetry in ii. directly above.
i. That was a gun.
ii. That (there) was a gun on the table.
In sum, and from a strictly structural view, the symmetry is as follows:
- It was a gun.
- It was a gun on the table. expletive-it
- It was a gun on the table. pronominal-it
- There was a gun on the table.
- That there was a gun on the table.