Hansson (1999; henceforth H) discusses an interesting case of defectivity in Icelandic imperative formation. According to H, this language has three types of (2sg.) imperative.

• The root imperative is available only as a “deliberate archaism”; it won’t be considered further.
• The full imperative consists of the root plus a coronal suffix plus a 2sg. pronominal enclitic -u /ʏ/.
• The clipped imperative also consists of the root plus a coronal suffix but uses a contrastively stressed pronoun ‘you’ (cf. English ‘YOU work!’) instead of a clitic.

For example, the full imperative for taka ‘to take’ is taktu [ˈtʰaxtʏ] and the clipped imperative is takt ÞÚ [tʰaxt ˈθuː].¹ H develops an account of the allomorphy of the dental suffix in the full and clipped imperatives; going forward I will cite the full forms, since the distinction is irrelevant. Under H’s analysis, there are two allomorphs:

• /-T-/ is a [−spread glottis] coronal obstruent surfacing as [t] or [ð] depending on context; e.g., the full imperative for negla ‘to nail’ is negldu [ˈnɛɣ͡ltʏ].²
• /-Tʰ-/ is a [+spread glottis] coronal obstruent, surfacing as [t] with devoicing of preceding stem-final consonants; e.g., the full imperative for synda ‘to swim’ is syntu [ˈsɪn̥tʏ].

H claims that “[f]or the vast majority of verbs, the choice of allomorph is uniquely determined on the basis of the root-final consonant(s)” (p. 108), implying that this is a phonologically conditioned allomorphy, though the conditioning is not given in prose form. H also implies (fn. 4) that this is suppletive allomorphy, though this assumption is also not justified. Let us assume, for sake of argument, that both assumptions are correct and this is a case of phonologically conditioned suppletive allomorphy. Finally, H notes that under his assumptions, there are certain roots for which either allomorph would give the same imperative surface form.

There are several exceptional verbs for which the phonological conditioning H proposes yields an incorrect result. For instance, the full imperative of senda ‘to send’ is the /-T-/ form sendu [ˈsɛntʏ] rather than the expected /-Tʰ-/ form *[ˈsɛn̥tʏ].³ H draws attention to weak verbs whose roots end in /ll, nn/. For these, H’s account of the phonological conditioning ought to prefer /-T-/, but most select /-Tʰ-/.⁴

There are four strong verbs whose roots end in /ll, nn/. So far, other than the characteristic ablaut, we have seen no reason to treat imperative formation in the strong verbs differently than in weak verbs.⁵ For example, for stela ‘to steal’, the full imperative is the /-T-/ form steldu [ˈstɛltʏ]. Yet, there are three strong verbs in /ll, nn/ for which neither possible form of the imperative is well-formed. These are the verbs vinna ‘to work’ (*vinndu, *vinntu), spinna ‘to spin (s.t.)’ (*spinndu, *spinntu), and falla ‘to fall; flunk’ (*falldu, *falltu). And to make matters more complex, there is one strong verb in /nn/ for which the “expected” /-T-/ is acceptable: the full imperative of finna ‘to find’ is finndu [ˈfɪntʏ].

H identifies the following explananda for imperative formation in Icelandic.

• The imperative stem is always the same as the past stem in weak verbs
• Yet, defectivity is found only in imperatives and never in pasts.
• Defectivity occurs only in strong verbs.
• Defectivity is found only in roots in /ll, nn/, a form which “usually is indicative of exceptionality in allomorph selection” (p. 344).

It is not obvious to me that the first explanandum is meaningful. While many linguists believe “Priscian”-like mechanisms which permit direct encoding of these kinds of facts, the mere stem identity of two semantically distant parts of speech is not itself compelling evidence. In this particular case, one might implement these facts without referring to identity by deriving the allomorphy from a verbal theme, perhaps a floating [α spread glottis] feature, which surfaces in both the imperative and the past. Thus roots selecting /-Tʰ-/ might be underlyingly someting like /√-ʰ/ where the surd denotes the root and /ʰ/ a thematic [+spread glottis] specification.

The second explanandum does seem to be meaningful, even independently of the first. One possible fact that might be relevant here is that (other than the enclitic) the Icelandic imperative is bare, whereas weak verb stems are, to my knowledge, always followed by a vowel-initial suffix. So one could imagine that this is, in part, a phonotactic effect at some level of prosodic structure that does not include the clitic.

The third explanandum also seems meaningful. One can, for instance, frame it as a simple statistical hypothesis test, the null hypothesis being that imperative defectivity is independent of the strong/weak distinction. While I don’t have psychologically plausible counts of the strong and weak verbs—the numbers I need to compute sufficient statistics for this test—in front of me, I suspect the probability of observing this pattern under the null hypothesis is going to be vanishingly small.

The fourth and final explanandum is certainly one worth incorporating into any analysis. However, I think the obvious step has not yet been taken: serious attempts out to be made to incorporate it into a phonological account of the coronal suffix allomorphy, something H unfortunately has not attempted. If we are in fact to regard verbs in /ll, nn/ as lexically exceptional, one should first reasonably exhaust possible phonological accounts. One direction for future research would be to better understand the allomorphy associated with the imperative and past stems in Icelandic in general.

H proposes, essentially, that defectivity results in strong verbs in /ll, nn/ because such verbs lack a coronal-suffixed past tense form elsewhere in the paradigm; he adds that the strong imperative finndu is exempted because there are other /…nt/ forms in the paradigm of that verb. So many, many, many different things have to go wrong for a defective imperative in Icelandic: essentially, one has to be imperative, in /ll, nn/, and lack other coronal-final stems, and this come together in just three verbs in the entire language. Whether or not one finds H’s account compelling, it is very difficult to reason much about the theory of defectivity from the existence of no more than three verbs in a language. We might do better to focus on languages, like Greek or Russian, in which inflectional defectivity has much higher type frequency.

Endnotes

1. Whether or not the full and the clipped imperative are pragmatically substitutable is unclear to me from H’s description.
2. Unfortunately, H does not always give the orthographic form of the words he is citing, and given the language’s famously difficult spelling, I am not always certain I have guessed the correct spelling for inflected forms. However, it appears to me that the contrast between /-T-/ and /-Tʰ-/ is spelled as -d- vs. -t-.
3. Once again, it is not clear why this is the expected form because the only description of the phonological conditioning is given in a sketchy Optimality Theory analysis (H:§2.1-2).
4. The relevant statistic is that 6 out of 33 weak verbs in /ll, nn/ select the “expected” /-T-/. From this H concludes that in this environment, “the exceptions far outnumber the regulars” (p. 113). I note briefly that under the tolerance principle (Yang 2005), an environment of 33 examples can tolerate up to 9 exceptions, so this could be a productive generalization according to that theory.
5. In H’s examples, strong imperatives use the same ablaut grade as the infinitive, so we just have to take his word that they are in fact strong.

References

Hansson, G. Ó. 1999. ‘When in doubt…’: intraparadigmatic dependencies and gaps in Icelandic. In Proceedings of NELS 29, pages 105-119. GLSA Publications.
Yang, C. 2005. On productivity. Language Variation Yearbook 5: 333-370.