What is this "it"? Is it like "it's sunny?"

Question

"It's off to work we go"

Is this "it" like "it's sunny."?

Answer 1

It's off to work we go.

There is either a relative adverb or a relative pronoun omitted in that sentence:

It's off to work where/that we go.

The 'it' is the subject, the 'is' (contracted) is the predicate. The prepositional phrase "off to work" is the predicative complement. The rest of the sentence is a relative clause, bound to the predicative complement ("off to work"), and in that clause "we" are the subject, "go" is the predicate.

The relative clause is used to explain something about the object of the main clause:

{I have a dog}_{main clause} {that likes his life}_{relative clause}.

In that sentence the 'dog' is the object, and the relative clause binds to it and explains something about it. The relative pronoun 'that' takes the role of the subject in the relative clause. "Likes" is the predicate.

Similar sentences (to yours):

{It was he}_{main clause} {who started the thread on ELL}_{relative clause}.
{It might be Susan}_{main clause} {whose book I've read}_{relative clause}.

In the former, "who started the thread on ELL" explains something about "he" subject complement. In the latter, "whose book I've read" explains something about "Susan".

I'm not sure there is a special designation for those, but they are pretty common.