I am trying to understand the grammatical structure of the following sentence, specifically the usage of "which":
The front door was open, which concerned me.
Is this a grammatically correct sentence? It seems one could re-write it as a main clause followed by a coordinate clause: "The front door was open, and that concerned me". Is "which" just used as shorthand for "and that"?
No, "which" is not used as short hand, and this construction is correct; "which" is here a relative pronoun with a special role; instead of standing for a noun phrase, as is often the case, it stands for a whole situation which has been expressed in what precedes. Such clauses introduced by "which" are called sentential relative clauses.
(CoGEL § 15.57) […] Unlike adnominal relative clauses, which have a noun phrase as antecedent, the sentential relative clause refers back to the predicate or predication of a clause, [1] and [2], or to a whole clause or sentence, [3] and [4], or even to a series of sentences [5]:
- They say he plays truant, which he doesn't. [1]
- He walks for an hour each morning, which would bore me. [2]
Relative clauses such as in [1] are used to affirm (if positive) or deny (if negative) an assertion or thought ascribed to others.
- Things then improved, which surprises me. [3]
- Colin marded my sister and I married his brother, which makes Colin and me double in-laws. [4]
In [3] the antecedent matrix clause is a single clause and in [4] it is two conjoined clauses. But one might equally imagine a storyteller coming to the end of the story with the words:
- which is how the kangaroo came to have a pouch. [5]
Here which could'refer back to the whole length of the story.
Yes, your sentence is correct.
In traditional grammar terms, which is a relative pronoun heading up a sentence relative clause here. Grammarian and English professor Richard Nordquist describes sentence relative clauses like this:
Sentence relative clauses refer back to the whole clause or sentence, not just to one noun.
Source: ThoughtCo — Relative Clause Definition and Examples in English
In your sentence—
The front door was open, which concerned me
—what concerned you was the entire preceding proposition, which was that the front was open.
The sentence is correct. Adding the word "which" adds some interest and mystery in the sentence. If we rephrased the sentence, we could remove the "which", and have a much flatter, less interesting sentence.
It concerned me that the front door was open.
That construction also puts the emphasis on "me" and not on the door, but it conveys the general idea.
We started out with a simple statement "The door was opened" but the "which" is part of a pronoun clause referring to the open door. It is renaming the phrase "open door" with concerns me.
