A correspondent writes to draw attention to this sentence:
The table was covered with objects, although once he removed the books, he was able to drag it to the centre of the room.
He asks: can a subordinating conjunction be used to connect the complex sentence beginning with once to the preceding simple sentence or can only a coordinating conjunction join compound-complex sentences? He suggests this alternative:
The table was covered with objects, but once he removed the books, he was able to drag it to the centre of the room.
The use of 'although', in cases like this, is certainly common in spontaneous speech. What we have is technically described as an anacoluthon, defined (eg by the OED) as 'a construction lacking grammatical sequence'. Such sentences work semantically, but at the expense of syntax, because they usually omit a required element. What's missing here is something like
The table was covered with objects, but this wasn't a problem because, once he removed the books, he was able to drag it to the centre of the room.
Anacolutha are able to occur because they rely on our semantic or contextual awareness to allow us to make short cuts in grammar. They're very frequent in spontaneous speech. There's no problem understanding such sentences, of course. There's no ambiguity. But they're frowned upon in formal writing, where (in the above example) the coordinate conjunction would be recommended. Many of the strictures in Fowler's Dictionary of Modern English Usage are against anacolutha of this kind.
No traditional grammar handles them - and even linguistic grammars pay scant attention to them. The realization that the grammar of speech is very different from the grammar of writing has become a big thing, over the past 50 years, but anacolutha remain one of the neglected areas of grammatical investigation. This is a shame, as they're so common in speech. I've just spent a mind-numbing few days listening over and over to a series of lectures I recorded last year, in order to write a commentary on them for a book/DVD package which Routledge are publishing in May this year, called The Future of Language. Because these were spontaneous performances, without written notes or an Obaman autocue, they contain several anacolutha. I draw attention to them in the commentary. Interestingly, the firm contracted to add the sub-titles regularized many of them, so that they conformed to written English norms. I had to change them back. WYS (in the sub-titles) IWYG.