Can "then" be used as a coordinating conjunction?

One of the best tests of a coordinating conjunction is its non-reversible order. We frequently place subordinate clauses beginning with the subordinating conjunction before the main clause, such as:

Because the man overate, he developed a stomach ache.

Coordinating conjunctions cannot be reversed in this way; HOWEVER (!!!)

The word "then" is often used alone, when in reality, in proper usage, it is "AND then". AND is the coordinating conjunction with THEN remaining and adverb of time, indicating WHEN the action occurred.

So, (an incorrectly placed coordinating conjunction... sorry), examining your original sentence:

He opened the door, then the lights came on and everybody shouted, ‘Happy Birthday’.

If we say,

He opened the door, AND then the lights came on, and everybody shouted.

We see that there are actually TWO independent clauses with AND as the coordinating conjunction, with THEN used properly as an adverb.

Hope this clears it up. Just because we don't say (or write) the AND, it is still (grammatically) there, the same way YOU is still the subject in imperatives, even though it is always omitted.

This article from Think Map - Visual Thesaurus addresses this issue [the version here slightly reformatted and condensed]:

Do This, Then Do That: Coordinating "Then" Usage_ Erin Brenner

Recently in an online forum for editors, someone balked at then being used as a coordinating conjunction, as in:

I went to high school, then I went to college.

Coordinating conjunctions, you'll recall, join two items of equal status: two words of the same parts of speech, two phrases of the same type (e.g. adverbial), or two clauses (independent or dependent). And, or, and but are coordinating conjunctions.

Look around, though, and you'll see then used as a coordinating conjunction with surprising frequency, even in professionally written and edited copy:

Rub the beets with 1 tablespoon olive oil, then add them to the pan.—The New York Times, 2012

I popped the cup into the microwave, set it to nuke anything unfortunate enough to be caught within its grasp for thirty seconds, then raided my fridge for sustenance.—Darynda Jones, Third Grave Dead Ahead, 2012

Given that these examples ended up in print, we need to ask why no one noticed the error.

Can then be used as a coordinating conjunction?

Defining Then

Then is commonly used as an adverb, adjective, or noun to indicate time:

Will you meet me then?

We contacted the then governor of Arkansas.

We'll meet again tomorrow; until then, review today's meeting notes.

Then is also used as an adverb to mean "besides," "in that case," and "therefore."

The American Heritage Dictionary was the only dictionary I found that addresses the question of then as a conjunction:

<< Sticklers for grammar sometimes assert that then is not a coordinating conjunction, and that the sentence She took a slice of pie, then left is thus incorrect; it must be rewritten as She took a slice of pie and then left, in which the then acts as an adverb and the halves of the compound predicate are linked by the coordinating conjunction and. But this use of then as a coordinating conjunction is actually both widespread and widely accepted; in our 2012 survey, more than three quarters of the Usage Panel found the sentence She took a slice of pie, then left completely acceptable.>>

The dictionary goes on to note that when then is used as conjunction, a comma is needed before it, which is different from how conjunctions like and function.

In 2012, I wrote about coordinating conjunctions and the criteria a word currently needs to meet to be considered one....

Then doesn't meet the full criteria of a coordinating conjunction. Like so and yet, it can be paired with and, but, and or:

I walked to the bus stop, but then I decided to take the train.

I will study all day, and then I'll take a nap, or then I'll take a walk.

While then can't be modified, it can only join specific types of clauses and the order of the clauses can't be switched, similar to so and for:

I will bring a notebook, then a pen. ≠ I will bring a notebook and a pen.

I walked to the bus stop, then I decided to take the train. ≠ I decided to take the train, then I walked to the bus stop.

As I noted in my conjunction article, The Cambridge Grammar of the English Language identifies so as 'like a conjunctive adverb, like however', and for as 'similar to subordinating conjunctions, like because'. If I had to categorize then, I'd say it's more like because and for, because the order of the items is generally important.

Using then

Then doesn't meet the specifications for a coordinating conjunction, but we're using it that way anyway — in professionally published copy, no less. As AHD points out, many of us don't notice or aren't bothered by then as a coordinating conjunction in certain conditions.

So can you use it as one?

If the use of then complies with the criteria discussed, you'll be in good company using it as a coordinating conjunction. If the usage doesn't meet the criteria or your audience isn't accepting of such a usage, you'd be wise to revise the sentence.

............

The 'if ... then ...' construction is, as far as I can see, always eligible for then-deletion.

The "part of speech" of a word describes the function that it has in a particular construction; but the same word can appear in different constructions. So there's no problem at all if then functions as an adverb in some constructions and as a coordinating conjunction in others.

One could argue, however, that then (when used as a coordinator) is prototypically subordinating, since the clause it modifies can have its subject omitted (linguists call this "subject pivot").

E.g.,

Thomas ate three hams, then started to feel ill.

Also note that the coordination places a semantic restriction on the two clauses: the first must refer to an earlier time (or metaphorically, to a logically prior state of affairs) than the second.

For example, the following sentence seems well-formed syntactically, but makes no sense.

?I forgot to do my homework, then Boniface reorganized the Frankish church.

This restriction will in some cases affect the possible grammatical tenses that can appear in the second clause, which is a good indicator of subordinating function:

*I will do my homework, then Boniface was about to reorganize the Frankish church.

In The Syntactic Phenomena of English, McCawley proposes Ross's CSC (Coordinate Structure Constraint) as test for whether a two part structure is coordinate. If a constituent cannot be extracted from just one part without being extracted from both parts, we classify it as coordinate, because it is subject to the CSC. Otherwise, if it is not subject to this constraint, it is not a coordinate structure.

For instance, from

You bought something for Joyce, and devoured it.  What was it?

by extracting both objects, we get

What did you buy for Joyce and devour?

but not

*What did you buy for Joyce and devour it?

Therefore, "and" is coordinate. Let's try it with "then".

You bought something for Joyce, then devoured it.  What was it?  
*What did you buy for Joyce then devour it?  
??What did you buy for Joyce then devour?

I conclude that "then" is coordinate (at least sometimes).

In fiction writing, using 'then' as a conjunction seems fairly standard nowadays. The only issue I see is that you are almost compelled to use a comma in a simple sentence with compound predicate, whereas with 'and' you wouldn't use the comma.

Example: We went to a restaurant and saw a movie. We went to a restaurant, then saw a movie.

Without the comma in the second example would lead to confusion (the possible meaning could be 'we went to a restaurant at that moment').