Ri-verbs
In traditional Icelandic grammar, (Icelandic: "ri-verbs") is a term that refers to 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 or -ð. These verbs are also the only verbs in Icelandic which inflect with the mixed conjugation except for the preterite-present verbs. Overview. The verbs are ("to heal, to grow"), ("to rub, to wipe"), ("to row") and ("to turn"). The principal parts of the ri-verbs are as following: 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" ("to sow") 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: 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: The forms with "ø" were older and resulted from a vowel rounding process (u-umlaut) 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" ("núa" in modern Icelandic) were adapted to the forms of "róa" and "snúa" by analogy, although they did not begin with "s-" or "r-" (their past tenses in Germanic were *"gegrō" and presumably *"gegnō"). 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.