The point we made was that combining clauses with “and” allows us to miss out the start of the second clause if it repeats the start of the first; we also provided a technical name for this process: “ellipsis”. A convenient way of showing this is by using the strikethrough font option:

“We have a hamster and we really want a goldfish.”

Like everything else in grammar, this topic deserves a second visit for added depth.

Ellipsis is fine with “and”, but terrible with “although”:“We have a hamster, although we really want a goldfish.” Why? Because “and” and “although” join the clauses in different ways. With “and”, the result is traditionally called a “compound sentence” because the clauses are co-ordinated - treated as grammatical equals. But with “although”, it’s a “complex sentence”, with one clause subordinated to the other.

Co-ordination allows ellipsis, but subordination doesn’t.

Ellipsis is a very useful grammatical tool. It avoids clumsy repetitions; eg look at these three ways of expressing the same idea: “Jimmy works hard and Jimmy contributes well in class.”

“Jimmy works hard and he contributes well in class.”

“Jimmy works hard and he contributes well in class.”

Each is an improvement on the one before it.

It’s not just subjects that can be elliptical; you can do it with anything at all at the start of the second clause that duplicates the first clause:

“Jimmy has unfortunately been playing too hard and he has unfortunately been working too little.”

But ellipsis isn’t just a matter of elegant style. In some cases, it’s essential to the meaning. Suppose you ask: “Who went out and who left the door open?”; how many culprits are you looking for? And how many in: “Who went out and who left the door open?”?

But however useful ellipsis is, it’s only co-ordinating conjunctions that allow it - not subordinating conjunctions. As we saw above, you can’t do it with “although”; so you can’t say: “Who went out to play although who hadn’t finished their homework?” - here you have to use “they”. It’s okay with “but”: “Who likes television but who finds films boring?” But it’s impossible with its near-synonym, “while”: “Who likes television while who finds films boring?”.

A useful activity would be to explore this distinction, collecting as many co-ordinating and subordinating conjunctions as you can. There are only about four co-ordinators but several dozen subordinators. This isn’t a matter of classifying words for the hell of it - those little words really matter.

Richard Hudson is professor of linguistics at University College, London.

Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk.