'Then' as adjective?

Yesterday, Nik Gisborne, mentioned off hand that English adjectives don't begin with /ð/ (the voiced "th-sound" at the beginning of there, but not thin). I offered then, as in his then wife, as an example and Nik suggested I follow up.

Here are the top ten th- adjectives from COCA. 

1		THIN	24158
2		THICK	22799
3		THEORETICAL	9392
4		THEOLOGICAL	6637
5		THOUGHTFUL	4443
6		THOROUGH	4255
7		THERAPEUTIC	3454
8		THERMAL	3125
9		THREE-YEAR	2894
10		THEATRICAL

All of them are voiceless /θ/ rather than voiced /ð/. This pattern continues until then appears in the 16th spot. The rest of the top 1,000 words also begin with either /θ/ or /t/, apart from some false positives, such as then-president or them-even. So, if then indeed qualifies, it appears to be the only such qualifier. But is it an adjective?

The OED hedges on whether then is actually an adjective, giving the following:

9b. attrib. or as adj. That existed or was so at that time; the then ruler = the ruler that then was.

Notice that it says "as adj.", rather than simply coming out and saying "adj." Presumably this is because apart from its location, there between the determiner and the head noun, it's quite unlike other adjectives: it starts with /ð/ for one thing, but it also fails the following tests:

1. modification by very (e.g., *the very then secretary)
2. grading (e.g., *the less then colonies)
3. complement function (e.g., *the president became then)

The entry in the OED helpfully suggests that we consult the entry for now, to wit:

D. adj.

a. Of or belonging to the present time. Common in the 17th cent.

No hedging there, perhaps because it's been around so long. Here are the earliest examples from the OED:

▸a1393   Gower Confessio Amantis (Fairf.) i. 2379 (MED),   Bot I am ferr fro thilke grace As forto speke of tyme now.

1444   Rolls of Parl. V. 75/1   The estate and possession of the saide nowe Maistur and Brethern.^†

In contrast, attributive then seems not to have shown up until much later:

1653   R. Baxter Saints Everlasting Rest (new ed.) ii. vi. §2 257   That the extirpation of Piety was the then great design.

Despite its long pedigree, and acknowledgement from the OED, attributive now does no better on the tests than then did.

1. modification by very (e.g., *the very now secretary)
2. grading (e.g., *the less now colonies)
3. complement function (e.g., *the president became now)

It seems like the only characteristics that these words share with adjectives is their ability to occur between a determiner and the head in an NP. So is there anything else that does this? I think an argument could be made for prepositions doing so. Consider the following examples:

the up side, the in side, the on button, a by-product, no through traffic, the after party, the out years, the against side, the before picture

The prefix over- might also be relevant here, but I'm just speculating.

Another apparent example is a like manner, but like really is an adjective here, not a pronoun.

On the other hand, the pattern doesn't seem to work with very many prepositions. For instance, I can't find or think of examples for any of the following: at, of, to, for, with, from, about, as, into. Nevertheless, I think it's fair to say that English does allow for PPs to functioning as attributive modifiers in NPs.

Of course, the OED calls then and now adverbs, not prepositions, but this is a mistake based on the idea that prepositions must have an NP object. I'm not going to go into that. If you're keen to learn about it, try here. So I conclude that, in attributive modifier function in NPs, now and then are prepositions. Nik was right.

Now the question is: why the hell doesn't English have adjectives that start with /ð/?



Notes
^†This is also a very early example of said in this attributive use. Here, it is prefaced with a determiner, but in the early 20th century, the determiner starts to be dropped from time to time, an issue I've taken up briefly here.