Desumasu

Desumasu is a term sometimes used to refer to the polite style of Japanese language, built from the copula desu and the verb ending masu. It is not a single grammatical category, but a shorthand way to describe how politeness is expressed across predicates in formal speech. Desu is the polite form of the copula and is used after nouns and na-adjectives to mark predication, while masu is the polite suffix attached to verb stems to indicate polite action in the present or future, with its own tense and negation forms.

In practice, desu marks noun or adjective predicates (for example, kore wa hon desu, That is a book), and masu marks verbs (for example, tabemasu, I will eat / I eat politely). The two can appear in the same discourse across different clauses, but in a typical sentence you use masu for verbs and desu for nouns or adjectives, maintaining overall politeness throughout the sentence.

Key forms include desu and deshita as the polite copula and its past form, and masu, mashita, masen, and masen deshita for verbs, to express present/future, past, and negative forms in polite speech. For example: watashi wa tabemasu (I will/eat politely), tabemashita (ate), tabemasen (do not eat), tabemasen deshita (did not eat). For predication with nouns or na-adjectives: sore wa hon desu (that is a book), sore wa hon deshita (that was a book).

Usage notes: This polite style is the default in many formal settings in modern Japanese, such as business, education, media, and customer service. It contrasts with the plain form used in casual speech among friends and family. See also Japanese grammar, keigo, teinei-go, and plain form.