Inflection Word Classes

In Japanese, are four parts of speech 品詞ひんし or word classes can inflect.

Inflecting Word Classes (Parts of Speech)
Word Type		Word Class (Part of Speech)
Independent Words 自立語じりつご	用言ようげん	verbs 動詞どうし	u-Verbs 五段ごだん
			iru-verbs 上一段かみいちだん
			eru-verbs 下一段しもいちだん
			ka-verbs カ変かへん
			sa-verbs サ変さへん
		i-adjectives 形容詞けけいようし
		na-adjectives 形容動詞けけいようどうし
Affix Words 付属語ふぞくご		Auxiliary Verbs 助動詞じょどうし

Verbs and adjectives are collectively called 用言ようげん (lit. "operable words" or "inflectable words"). All 用言 can be used independently (自立語じりつご) in a sentence. Auxiliary verbs, however, are affix words (付属語ふぞくご) and must attach to another word to be useful – they are never independent.

Anatomy of Inflection

All inflecting words change form to express nuances such as tense, aspect, mood, polarity, etc., all while retaining a basic meaning.

Usually a portion of the word remains unchanged – this part called the word stem or base, or 語幹ごかん in Japanese.

Japanese inflections always occur at the end of a word. The part of the word that changes through inflection is called the inflection tail, or 活用語尾かつようごび in Japanese.

Inflected Word Anatomy
Word		語幹ごかん Stem		活用語尾かつようごび Inflectional Tail
美うつくしい	＝	美うつくし	＋	い
話はなす	＝	話はな	＋	す

Technically speaking, all four of these word classes are a form of verb. Because of this, what we're really talking about is "conjugation" (活用), which is a subclass of "inflection" (屈折) that applies to verbs.

Inflection & Agglutination

Very few useful Japanese stem + inflecting tail combinations are useful by themselves.

Japanese is both (1) inflectional and (2) agglutinative. Inflection was explained above. Agglutination is the process of affixing (adding) one or more words to a base word, like connecting train cars to a locomotive. Through agglutination, new meanings can be created.

Some of the agglutinating affixes can themselves inflect and except further affixes. But words don't necessarily have to inflect in order to agglutinate.

Inflection and Agglutination
Inflection				Agglutination
語幹ごかん Stem		活用語尾かつようごび Inflectional Tail		Affix
美うつくし	＋	かっ	＋	た
話はな	＋	し	＋	ます

The illustration above shows how inflection and agglutination can be used together. These are two simple examples, how there are many combinations for words that can affix to inflecting words:

Particles such as に・は・も etc.
Inflecting and non-inflecting auxiliary verbs such as う・て・ない
Other parts of speech, e.g. other 用言 or nouns