The most notable function of this root was as the basis for the Quenya genitive suffix -o. The introduction of this root seems to predate the later iteration of this form of the Quenya genitive, with the root first appearing in The Etymologies of the 1930s as ᴹ√ƷŌ̆ where it served as the basis for the prepositions ᴹQ. ho/N. o “from” as well as an “old partitive” ᴹQ. -on (Ety/ƷŌ̆). In 1930s the usual Quenya genitive suffix was ᴹQ. -(e)n as in ᴹQ. Nyarna Valinóren “Annals of Valinor” (LR/202). However, the genitive suffix was often -o in earlier iterations of Quenya, and even in the 1930s it was a feature of the genitive plural, as in ᴹQ. ar antaróta mannar Valion “and he gave it into the hands of the Lords (Vali-on)” (LR/72).
After the conceptual shift of the primitive velar spirant from ʒ to h, this root became √HO (WJ/361), and Tolkien discussed its function at length in the Quendi and Eldar essay of 1959-60. He said it “was evidently an ancient adverbial element, occurring principally as a proclitic or enclitic: proclitic, as a prefix to verb stems; and enclitic, as attached to noun stems” (WJ/368). As proclitic it formed the verbal prefix Q. hó- “away, from, from among”, as an enclitic it formed the basis of the noun inflectional suffix -o used for genitives in Quenya of the 1950s and 60s. Its role in Sindarin is less clear, given Tolkien’s later vacillations on whether or not suffixal genitives were even a part of Sindarin.