Manyogana
Writing characters by themselves is often okay in Chinese, since Chinese is an analytic language, or a language where grammar is structured by word order, but Japanese is highly inflectional, meaning that grammar is structured by word endings. For that reason, verbs, adjectives, and adverbs almost always have at least two morphemes, the first being the derivational morpheme, or the part of the word with the root meaning, and the second and others after it are called inflectional morphemes, or parts of the word added to show a word's grammar. For example, a word that uses the character 見 is 見る mi(ru),  where the first morpheme, the kanji being 見 and its pronunciation being mi, has the root meaning "see", the second syllable ru shows that the word is a verb, making it into the word "to see". The adjective 多い oo(i) is made up of the morpheme 多 oo, meaning "many", and the morpheme い i, showing that the word is an adjective. The last examples are the two words 速い haya(i) and 速く haya(ku). While the first morpheme in both words are the two syllables 速 haya, meaning "fast",  the い i　in 速い makes the word into an adjective while the く ku in 速く hayaku turns the word into an adverb, thus the word becomes "quickly".