The book that I am using says a form of qui is used at the beginning of a sentence in order to “tie the sentence more closely to the one preceding” but offers no further explanation. Is it correct in this case to observe the grammatical rules of the relative pronoun?
The relative pronoun in this situation behaves just like a relative pronoun at the head of a relative clause: it agrees in gender and number with its antecedent in the preceding sentence, and its case is determined by its function in the sentence of which it is a part. In this use, the relative pronoun is equivalent to a demonstrative or personal pronoun.
I think this is more common in later/ecclesiastical Latin (somewhat colloquial?), but I’m not sure about that. Here is an example given in A Primer of Ecclesiastical Latin: Discipuli in domum veniebant. Qui cum laetitia Jesum audiebant. This is basically the same thing as saying Hi cum laetitia Jesum audiebant.
I wouldn’t say it’s more common in later or ecclesiastical Latin. You’ll find this all over the place in Caesar. And I don’t think it’s colloquial either–it’s widely used to connect sentences in a formal narrative style.
I stand corrected then. I read mostly EL and come across it all the time and assumed it was a feature of EL. Much of the CL I read is not narrative, and the vast majority of EL I read is narrative, so that may be the reason I’ve thought that.
Grammatically it’s quite an interesting phenomenon, for it subordinates its clause to what precedes it rather than starting a new sentence, so it shouldn’t really have a full stop before it. (A dash might serve—but then I’m overfond of dashes.) Even though it can seem to make a distinct break (e.g. quae cum ita sint, or QED), it nonetheless effects continuity.
Which I take to be the whole point of it.
I was thinking somewhat the same thing, that it’s really in a way still a subordinate clause although not adjectival as relative clauses are. The example I gave could be re-punctuated as: Discipuli in domum veniebant, qui cum laetitia Jesum audiebant. However, Discipuli, qui cum laetitia Jesum audiebant, in domum veniebant changes the meaning. The former implies a sequence of events, the latter doesn’t.