In
Japanese grammar (depending on who you talk to), the base is what links the
verb stem with the
verb ending to form the complete
verb. There are basically seven bases, and the system for forming bases depends on the type of
verb and in some cases what
syllable it ends with.
Japanese verbs have a conjugation system totally different from that of English or any other Western language. Japanese moves a great deal of meaning out of linking/helping verbs and grammatical auxiliaries and into the verb. For that reason, Japanese verb usage is best described in terms of bases and endings.
Base 1 is sometimes known as the "Negative Base", because most of the endings used with Base 1 have some negative aspect. This base is required to express concepts such as "without (doing something)", conditional negatives, negative requests, obligations, etc. Base 1 calls for an ending; it cannot be used by itself.
Base 2 used by itself changes a verb to a noun. Also, using a verb in Base 2 with no ending and adding the word mono (thing) forms a new noun that means "things to (verb)", such as tabemono (things to eat) or kaimono (things to buy, i.e. shopping). Base 2 can be used with no ending.
Base 3 is the dictionary form of the verb. It is also the plain present tense (as opposed to the polite present tense, which is more useful for foreigners). Base 3 can also be used with no ending.
Base 4 can be used with several conditional-related endings. Also, Base 4 by itself can be used for abrupt commands, such as "Yame!!" (Stop it!!). Notably, the potential forms (can, can't, able to, etc) require Base 4.
Base 5 is less common than the other bases and few endings require it. The main use is for the informal inclusive command, such as "kaeroo, ne" (let's go home) or the informal inclusive query, such as in "asu mata koyoo ka" (shall we come again tomorrow?).
Base 6 is an important base that is often referred to in textbooks as the TE-form of the verb. Base 6 is used all over the place. With no ending, it allows verbs to be used in series. Example:
Uchi ni kaette, ban-gohan o tabemashita.
I returned home, then had dinner.
Base 6 is used with many common endings, like the desiderative (want), polite request, progressive forms, etc.
Base 7 also has wide usage. By itself it forms the plain past. Endings used with Base 7 aren't easy to classify, but they include cause-and-effect, informal listing of verbs, the concept of just having done something, and others. Base 7 is a major staple of informal speech.
Japanese "verb endings" consist of suffixes, which result in a new word, or additional words which directly follow the verb in the sentence and alter the sense of the verb. For example, the familiar -masu is a suffix ending, but koto ga aru is a common word ending (meaning "had the experience of"). There are many, many possible endings overall.
See the individual bases for information on how to form the base for different verbs and selection of endings. Base-forming information is also given in the individual verb writeups.