Unlike most European languages, Japanese has an extensive grammatical system to express politeness and formality.
Broadly speaking, there are three main politeness levels in spoken Japanese: the plain form (kudaketa), the simple polite form (teinei) and the advanced polite form (keigo).
Since most relationships are not equal in Japanese society, one person typically has a higher position. This position is determined by a variety of factors including job, age, experience, or even psychological state (e.g., a person asking a favor tends to do so politely). The person in the lower position is expected to use a polite form of speech, whereas the other might use a more plain form. Strangers will also speak to each other politely. Japanese children rarely use polite speech until their teens, at which point they are expected to begin speaking in a more adult manner.
The plain form in Japanese is recognized by the shorter, so-called dictionary (jisho) form of verbs, and the da form of the copula. In the teinei level, verbs end with the helping verb -masu, and the copula desu is used. The advanced polite form, keigo, actually consists of two kinds of politeness: honorific language (sonkeigo) and humble (kensongo) language. Whereas teineigo is an inflectional system, keigo often employs many special (often irregular) honorific and humble verb forms.
The difference between honorific and humble speech is particularly pronounced in the Japanese language. Humble language is used to talk about oneself or one's own group (company, family) whilst honorific language is mostly used when describing the interlocutor and his group. For example, the -san suffix ("Mr.", "Mrs." or "Ms.") is an example of honorific language. It should not be used to talk about oneself. Nor should it be employed when talking about someone from one's own company to an external person, since the company is the speaker's "group".
Most nouns in the Japanese language may be made honorific by the addition of o- (お) or go- (ご); as a prefix. o- is generally used for words of native Japanese origin, whereas go- is affixed to words of Chinese derivation. In some cases, the prefix has become a fixed part of the word and is included even in non-honorific speech, such as gohan, or rice. Such a construction usually indicates deference to either the item's owner or to the object itself. For example, the word tomodachi ("friend"), would become o-tomodachi when referring to the friend of someone of higher status. On the other hand, a female speaker may sometimes refer to mizu (water) as o-mizu merely to show her cultural refinement, compared to more abrupt male speech patterns.
Many researchers report that since the 1990s, the use of polite forms has become rarer, particularly among the young, who employ politeness to indicate a lack of familiarity. That is, they use polite forms for new acquaintances, but as a relationship becomes more intimate, they speak more frankly. This often occurs regardless of age, social class, or gender.