I understand that polite forms like yomimasu are actually (or originated as) infinitive yomi + auxiliary verb masu in its plain non-past form. Then past form yomimashita is just infinitive yomi + plain past mashita.

So I wonder what is the origin of the negative form, such as yomimasen. Given the above, one might naively expect a plain non-past negative form of masu, which would give yomimasanai. Is -masen "explainable" in terms of other forms? Is it a contraction from an originally longer form?


According to Shogakukan's big 国{こく}語{ご}大{だい}辞{じ}典{てん}, the verb ending -masu ultimately derived from a combination of humble polite auxiliary verb 参{まい}る plus the verb する, as a shift from either ‑mairasuru or possibly ‑maisuru. The final ‑su in modern ‑masu conjugates identically to classical su / suru. The 未然形{みぜんけい} ("incomplete form") that is used to form negatives ends in ‑a in most classical 四段{よだん} verbs (which became modern 五段{ごだん} or "type 1" verbs), such as kaka‑ as the mizenkei for 書{か}く, or ika‑ as the mizenkei for 行{い}く, etc. However, classical su / suru is irregular, and its mizenkei is se‑ instead. (This is still the mizenkei for suru in some modern dialects, such as Kansai-ben.) So the negative of ‑masu was formed as the mizenkei of ‑mase‑ plus the negative ending -nu, as ‑masenu. Final ‑nu contracts to just ‑n, giving us modern ‑masen.

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