Q: Your post about “needs must” is very interesting, but try as I might I find it hard to construe “nights” and “days” as adverbs in “She works nights and sleeps days.” They just feel too like nouns, being the object of “works.” Can you give any other examples of “-s” and “-es” adverbs in Old English?
A: Let’s begin with a brief overview of how adverbs were formed in Old English.
Some began life as adverbs, including a few that still look much as they did in Anglo-Saxon days:
hér (“here”), oft (“often”), sóna (“soon”), þæ’r (“there”), þonne (“then”), hwílum (“sometimes”), and so on. (The þ, or thorn, and ð, or eth, were Old English versions of “th.”)
But the majority of adverbs were formed by adding the suffix -e to adjectives, according to The Origins and Development of the English Language (4th ed.), by the linguists Thomas Pyles and John Algeo.
So déop (“deep”) became déope (“deeply”), wid (“wide”) became wide (“widely”), fæst (“fast”) became fæste (“fast,” adv.), and so on. However, most of these -e suffixes disappeared by the 14th century.
Many other adverbs were formed by adding the suffix -lice to adjectives:
beald (“bold”) became bealdlíce (“boldly”), swét (“sweet”) became swétlíce (“sweetly”), and so on.
Over the years, the -lice adverbs evolved into the modern “-ly” ones. And as the -e suffix died out, “-ly” became the usual suffix for turning adjectives into adverbs.
Still other Old English adverbs—the ones you’re asking about—were formed by adding the suffix -s or -es to nouns: þanc (“a kindly thought”) became þances (“thankfully”), for example, while sóþ (“truth”) became sóðes (“truly” or “forsooth”), and endebyrd (“arrangement”) became endebyrdes (“in an orderly manner”).
(The -es suffix here is the same as the genitive singular ending on many neuter and masculine nouns in Old English. The genitive, as you know, is a case expressing possession and similar relationships.)
An interesting example is the Old English term word, which inspired the adverb wordes (“verbally” or “orally”). Although the noun word has survived intact in modern English, the adverbial sense of wordes seems to have died out in Old English. (“Verbally” comes from verbum, classical Latin for “word,” while “orally” comes from ōs, classical Latin for “mouth.”)
Pyles and Algeo, in their comments on -s and -es adverbs in Old English, also cite hámweardes (“homewards”) and tóweardes (“towards”), and note that the “same ending is merely written differently in once, twice, thrice, hence, and since.”
Some Old English and early Middle English adverbs with -s or -es suffixes adopted –st endings in Middle English, according to the OED. The Old English ongegnes, for example, eventually became “against.” A similar process occurred with “amidst,” “amongst,” and “betwixt.”
We should mention here that this is a highly simplified description of a very complicated and messy process.
For instance, many adverbs had all three suffixes (-e, -es, and -lice) in Old English. And “against,” “amidst,” “amongst,” and “betwixt” were originally compounds. For example, “amidst” began as a + midde + -s. And many Old English adverbs looked the same as adjectives and prepositions.
We see your point about “nights” and “days,” but the OED and most standard dictionaries we’ve checked classify them as adverbs when used to modify verbs.
from Grammarphobia http://www.grammarphobia.com/blog/2017/02/an-adverb-forsooth.html