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I looked through the dictionary and found that the pronoun "it" has may usage:
1.Used as the subject of an impersonal verb:

eg. It is snowing.

2.Used as an anticipatory subject or object:

eg. Is it certain that they will win?

3. Used to refer to a general condition or state of affairs:

eg. She couldn't stand it.

But in the usage below, what dose "it" used as?

it's good to talk to you 

it appears that ....

it seems as if .....

it sounds as if ...

it is (not) as if ....
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copysnakeBut in the usage below, what dose is "it" used as?
They are all examples of 2. (anticipatory subject). The ones with "as if" could be 3. in certain contexts. Context is always important. Without it, it can be impossible to determine what you're dealing with.

CJ
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But if "seem" "sound" "is" are link verbs, the clause "that..." the phrase "as if..." should be Predicatives i think.
And if thery are real subjects, can a link verb exist without a predicative?

One more question, Can pronoun "it" be a subject when there is no mention of what it refers to?

Thank you!
copysnakeCan pronoun "it" be a subject when there is no mention of what it refers to?
Yes. Non-referring it, or dummy it. That's what "anticipatory it" is. Just different names for the same thing.

It is certain that Joe finished the task.
=
That Joe finished the task is certain.


This is an obvious case of anticipatory it (i.e., dummy it)

Here's another:

It is important to take an umbrella today.
=
To take an umbrella today is important.


The real subject is moved to the end, and the word it is placed where the subject used to be.

CJ
copysnakeBut if "seem" "sound" "is" are link verbs, the clause "that..." the phrase "as if..." should be Predicatives i think.
And if thery are real subjects, can a link verb exist without a predicative?
You got me there. I don't have a good explanation for this one. I would call this a case of "impersonal it". The reference is to something vague and general -- the situation in general. This is somewhere between your descriptions 1. and 3.

It seems as if ...
It sounds as if ...

CJ
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