Preparatory it; not possible for complements

I was reading Practical English Usage, by Michael Swan and got into something that has got me deeply confused. It basically says that preparatory it can be used as a preparatory subject or object, but not as a preparatory complement. Firstly, I didn't even know that English made a difference between complement and object, nor had I ever found a context in which I would really care. I searched online and I think I understand the difference now, but the issue with "it" still makes me want to cry. The examples the book uses are: -This chair is comfortable to sit on. (not It is comfortable to sit on this chair). (to sit on is the complement of comfortable.) They both sound fine to me, although I think I get that that "it" is making comfortable a subject. Here I ask: what about This chair is to sit on/ It is to sit on, this chair. I know they sound weird, but I use them as an example, as I want to understand the grammar behind it. -The impression was given that travel expenses would be paid. Here expenses is the complement of the impression).

As you see, both these examples contain complement objects, not complements that affect directly the subject (which are far more common and easy to grasp, I believe).

So, how does this work? Could you give me some more examples that make the issue clearer? What about, say... "The chair is wonderfully beautiful"; "It is a wonderfully beautiful chair"; "It is wonderfully beautiful, the chair".

PS: I am confused/ it confuses me/ it is Confucius.

Thanks in advance!

Solution 1:

Your examples use linking verbs, for example:

"It is a wonderful day."

Linking verbs don't have objects. They only have subjects and/or complements. That means "a wonderful day" isn't the object of the verb "is" in the above example; instead, it's the complement, a second subject of the verb "is," both the preparatory "it" being a complete subject of "is" and "a wonderful day" being another complete subject of "is."

When what you're referring to tells you that a preparatory "it" can't be used as a complement, it's essentially pointing out that "A wonderful day is it" doesn't make sense as a declarative sentence (i.e., "A wonderful day is it."). The preparatory "it" loses any meaning when it is moved to the complement.

Now, as an interrogative sentence (i.e., "A wonderful day is it?), it works, but that's because the preparatory "it" isn't actually the complement. Interrogative sentences use the inverted structure that questions use of placing the verb before the main subject, so "it," despite coming after the linking verb (i.e., "is"), isn't the complement when it's written as a question. Also, to especially point out that "a wonderful day" is the complement is why you would very often see that question written with a comma (i.e., "A wonderful day, is it?"), despite the fact that the comma separates an adjacent subject from its verb, something that is generally ungrammatical.