[This is part of a small but growing series of defectivity case studies.]

Swedish has two genders: a common (or uter) and a neuter. The uter form consists solely of the adjectival stem, whereas the neuter is formed by appending a suffix normally spelled -tt. This suffix, by hypothesis /-tː/, triggers voice assimilation, degemination and/or vowel shortening in some stems. For instance, the neuter form of röd [røːd] ‘red’ is rött [rœt]: here /…d-tː/ is realized as just [t] as the result of assimilation and degemination, and long /øː/ is shortened to short (and lower) [œ].

However, not all adjectives have a well-formed neuter (e.g., Hellberg 1972, Eliasson 1975, Iverson 1981, Löwenadler 2010). Some of the defective categories, after Löwenadler, are:

• Both monosyllabic adjectives ending in a short vowel followed by -dd: fadd ‘stale’, and rädd ‘scared’. (However, Hellberg notes that neuter past participles, which have the same surface form, are well-formed: thus fött is the well-formed neuter past participle of föda ‘to feed’. Presumably the past participle formative /-d-/ is treated differently than stem-final /-d/.)
• Certain monosyllabic adjectives with long vowels ending in -t or -d: lat ‘lazy’, flat ‘ibid.’, kåt ‘horny’, rät ‘straight’, pryd ‘prudish’, vred ‘wrathful’, snöd ‘vile’.
• Most polysyllabic adjectives in -d with final stress, many of which are borrowings from French: morbid ‘ibid.’, hybrid ‘ibid.’, rapid ‘ibid.’, gravid ‘pregnant’, timid ‘ibid.’. (However, Hellberg reports that solid ‘ibid.’ has a neuter: solitt [sulitː] is apparently well-formed.)
• Adjectives ending in a stressed vowel: disträ ‘absent-minded’, blasé ‘ibid.’, kry ‘healthy’.

As with Norwegian, I am left wondering whether there are other places in Swedish grammar where -dd affixation might lead to ineffability. Eliasson (1975) and Iverson (1981) claims that verbs in -dd never follow the second or third conjugation, in which certain cells would pose similar problems to the neuter adjectives. Instead such verbs all belong to the first conjugation, which has a theme marker -a- which avoids this issue.

It also seems that the wellformedness of solitt will be an important point for any final theory. There is clearly some individual variation too, as documented by Löwenadler (2010).

Other theoretical accounts of this phenomena, which I didn’t find much to say about, include Buchanan 2007, Lofstedt 2010, and Raffelsiefen 2002.

References

Buchanan, C. H. 2007. Deriving asymmetry in Swedish and Icelandic inflexional paradigms. Master’s thesis, University of Tromsø.
Eliasson, S. 1975. On the issue of directionality. In K.-H. Dahlstedt (ed.), The Nordic Languages and Modern Linguistics 2, pages 421-455. Almqvist & Wiksell.
Hellberg, S. 1972. Ordering relations in the phonology of Swedish adjectives. Gothenburg Papers in Theoretical Linguistics 13: 1-16.
Iverson, G. 1981. Rules, constraints, and paradigm lacunae. Glossa 15: 136-144.
Lofstedt, I. P. M. 2010. Phonetic effects in Swedish phonology: allomorphy and paradigms. Doctoral dissertation, University of California, Los Angeles.
Löwenadler, J. 2010. Restrictions on productivity: Defectiveness in Swedish adjective paradigms. Morphology 20: 70-107.
Raffelsiefen, R. 2002. Quantity and syllable weight in Swedish. Ms.