Ri-verbs

In Icelandic grammar, the ri-verbs are the four verbs in the language that have a -ri suffix in the past tense as opposed to a suffix containing a dental consonant such as /d/, /ð/, or /t/. Along with the, they are the only verbs which inflect with a

Overview

The verbs are wikt:en:gróa#Icelandic, wikt:en:núa#Icelandic, wikt:en:róa#Icelandic and wikt:en:snúa#Icelandic.
The principal parts of the ri-verbs are as following:

First principal part	Second principal part	Third principal part
Infinitive	First person singular past tense indicative mood	Past participle
Að snúa	Ég sneri or snéri	Ég hef snúið
Að gróa	Ég greri or gréri	Ég hef gróið
Að núa	Ég neri or néri	Ég hef núið
Að róa	Ég reri or réri	Ég hef róið

The spelling sneri reflects the original pronunciation of these words, while snéri reflects the modern pronunciation. The Icelandic Ministry of Education considers both variants to be equally correct.

Origin

Historically, róa and snúa belonged to the seventh class of "strong" (irregular) verbs, which was the only class of verbs in Germanic that had retained the reduplication inherited from the Proto-Indo-European perfect aspect. In Old Norse, the verb sá also belonged to this group, but it has become regular in Modern Icelandic. The past tense of these three verbs from Proto-Germanic and Proto-North-Germanic was as follows:

*rōaną - *rerō
*snōaną - *sesnō > *seznō
*sēaną - *sesō > *sezō

Originally, all conjugation class 7 verbs showed this reduplication. In most verbs containing -ē- in the stem, this changed to -ō- through a process known as ablaut, which was common to all strong verbs. The change from s- to z- was due to Verner's law, a historical sound change in the Proto-Germanic language whereby voiceless fricatives were voiced when immediately following an unstressed syllable in the same word. Given that reduplicating prefix was originally unaccented, this caused voicing of /s/ to /z/. In Old Norse, this -z- was rhotacized to -r-, creating the following forms:

italic=no - italic=no, italic=no
italic=no - italic=no, italic=no
italic=no - italic=no, italic=no

The forms with ø were older and resulted from a vowel rounding process caused by word-final -ō, which became -u in Old Norse before it was deleted altogether. Following this, the verbs adopted the endings of irregular verbs in the past tense, with -a, -ir, -i in the first, second and third person singular past, and later the original vowel e was restored. The verbs gróa and gnúa were adapted to the forms of róa and snúa by analogy, although they did not begin with s- or r-.
In modern Icelandic, the first person singular ending was replaced by -i in all weak verbs, and the ri-verbs followed suit. The verb sá then eventually became weak, reducing the number of ri-verbs to the current four.