Conjugations bases are the "true" forms in which Japanese verbs and i-adjectives can be conjugated. They each convey a certain nuance, different from what English verb conjugations mean. Some of these bases can be used by themselves, especially in Old Japanese. In modern Japanese, these bases mostly important for the compound conjugation, which expresses meaning such as tense, mood, attidude of speaker and much more by adding suffixes, verbs and adjectives.
Literally, "ending form", also called predicative form. This is the classical form used in Classical Japanese for ending sentences, hence its name. Dictionaries before Modern Japanese often listed verbs and adjectives under this form. In modern Japanese, this form was replaced by 連体形 (see below) and is only used to make sentences sound more literary/archaic/fancy.
酔(ゑ)ひも為(す) (yoi mo su) Also see illusions. す is the 終止形 of する
Literally, "continuing body form"attributive form. In classical Japanese, it was only used for attributing a sentence to a noun, for example:
人 病気で 死ぬる (終止形) [A human dies of illness.] 病気で 死ぬ 人 (連体形) [A human who dies of illness.]
The first sentence uses the 終止形 to form a simple sentence, the second sentence attributes the sentence "to die of illness" to "human", literally translating to "a 'dying of illness'-human".
Modern Japanese, however, uses this base instead of the 終止形 for ending sentences as well. In modern Japanese dictionaries it verbs and adjectives are thus listed under this form, hence its alternate name "dictionary form".
It should be noted that although the 連体形 is sometimes described as a "base/basic form" or "infinitive form", there is nothing more basic or inifinite to the 連体形 than to other forms. Verbs in Japanese do not have an "infinitive". It is simply convenient to agree on a base when speaking of a verbs plain form.
It is interesting to understand why this base replaced the 終止形:
Firstly, due to the fixed word order when attributing a sentence to a noun and that the verbs always comes last to end sentences, there is no special need to distinguish between a predicative (終止形) and attributive (連体形) form. No confusion can arise. From the viewpoint of convenience and making things uncomplicated, it is reasonable to expect that in normal conversation, this distinction is lost. Indeed, the colloquial language that later became modern Japanese did just this.
Secondly, there may have been more ways of doing this. In the end, common usage agreed on one way:
As a remnant of old Japanese, the 連体形 could also a nominal meaning (used as noun). The particles が originally was quite like の nowadays, linking to nouns. Thus saying, instead of
which uses the 終止形 normally,
人が死ぬ (the dying of a human)
expresses the same meaning within the bounds of classical Japanese grammar. This is also the explanation why が is used to mark the subject in Modern Japanese.
Literally, "continued-use form", also called "continuative" or "conjunctive" or "adverbial". This base is used to, in general, link words or sentences together.
Used to link sentences, it connects them in a loose way:
花咲き、鳥鳴き、春が来る flowers are blooming, birds chirping and spring coming
With adjectives, it links them all to a noun:
赤く長く大きいペン a red, long, big pen
Sometimes, for the words it connects, one stands in a modifying relation:
敬い申す to speak reverently 恋しく思う to think lovingly
The relation between the words can be quite complex:
焼き殺す burn to death 醜い (from 見難い) hard to look at
So it is best to simply remember that it connects words and have a guess using context.
This is also the form frequently used with helper verbs/adjectives to express many shades of meaning:
-ます polite form -たい want to ~ -たり listing -た/き past -つ/ぬ affirmative -くる gradually start to -いく gradually stop to -はじまる begin to -治す redo / do again &c.
It also serves as a noun form of verbs in Japanese, in which case it means "the act of doing [verb]". It can also be joined with other nouns.
使い方 (lit. "using-way") instructions
Literally, "uncompleted state form", also called Imperfective (not to be confused with the Imperfect in languages such as French), opposite of 已然形. This base is used for things not yet completed or done. It it used for conditions, the future, negation, passive verb/potential verb and causative verb. In detail:
In classical Japanese it is used with ば for a yet unrealized condition:
読まば if/should he read(s)
A remnant of this usage in modern Japanese can be seen in the 未然形 of たり:
御金を持ったら新しい車を買います If I had money, I would buy a new car.
It is used for the future form in classical Japanese using the helper verb む:
我(わが)世(よ)誰(たれ)そ/常(つね)成(な)らむ (イロハ) What in this world / Will remain as it is?
As well as for the "pseudo-futurum" in modern Japanese, which is just the form above with some sound changes:
成らむ > 成らう > 成ろう
Further, it is the base used for the negation:
飲まず (not drink) 有りません (not exist) 座らない (not sit)
It appears to be used for the passive/potential form, however, this is probably due to a sound change and originally used the 連用形. See Potential verb
Lastly, it serves as a base for forming causative verbs:
サカヤシメ タマエ deign to cause (them) to flourish メ ノ ヲムナ ニ アヅケテ ヤシナワス (Taketori Monogatari) he gave it (the child) to his wife to bring it up 話させる cause to talk
This is the only base that cannot be used on its own suggesting that it may be composite. However, as there is not much known on the etymology and origin of the 未然形, this is only speculation.
Literally, "completed state form", also called Perfective, opposite of 未然形. In classical and especially Old Japanese used as a form for completed actions. It could be used on its own, for example
我(われ)忘(わす)るれや Have I forgotten? (Manyoushuu)
As a related sense, it was also used to express reason or a "completed condition":
hotoke no ... sakiwaetamau mono ni ari to omoe (because we think the Buddha was a thing bestowing blessedness)
This can be thought of as "having thought".
In modern Japanese, this base is only used for yet unrealized conditions (formerly expressed by the 未然形), which is why it is also called 仮定形 (assumption form). In this use, modern Japanese adds the particles は in its voiced form ば. Note that "unrealized condition" here means not "if X should be the case", but "when/the moment that X is the case". The conditions is yet unrealized, although quite possible. For example:
御金を貰えば新しい車を買います When I get the money, I will buy a new car.
Observe the difference:
行(ゆ)かば (未然形) if he goes / should go 行(ゆ)けば (已然形) since he goes / has gone
Lastly, in Classical Japanese it was also used in
行(ゆ)けど though he goes / has gone
do express a restriction (concessive).
Literally "commanding form". Etymologically, this may simply be a special usage of the 已然形. It is only a small step from saying "as a fact, verb action is completed" to "verb action has to be completed" or "I want verb action to be completed."
This form expresses a very direct command. For 五段活用動詞, this form is used on its own, for 一段活用動詞 the suffixes よ or ろ and others in certain dialects are added to express a command. These suffixes had origninally only an emphatic meaning (do! verb action).
行け!(Go!) 食べろ! (Eat!) 見よ! (Take a look!)