On the inflectional ending -s in English

There are two types of inflection in Present-day Standard English: “declension” for nouns and adjectives, and “conjugation” for verbs. Of them the inflectional ending -s functions as allomorphic markers that represent the plural number or the possesive/genitive case of nouns, and is also used for verbs in the present indicative third-person singular form. This fact means that in speech it has ambiguity in nouns and can be ambiguous between nouns and verbs, in case they are phonetically identical. Historically speaking, this -s has derived from (1) -as, which was the nominative/accusative plural ending for strong masculine nouns in Old English (OE), (2) -es, which was the genitive singular ending for strong masculine/neuter nouns in OE, and (3) -es, which was the present indicative third-person singular ending for verbs in northern and midlands dialects in Middle English. The reason why this -s has survived without being eroded like other endings is thought to be the phonetically conspicuous sound it uniquely has: <s>, which other endings did not have with them. We English learners, therefore, especially when these endings are used in speech, with the historical facts in mind, must specify the environments where they function so that we can avoid the ambiguities they could cause.