The adoption of Chinese characters has proven to be the equivalent of a linguistic Procrustes bed in the case of Japanese. A highly inflective language like Japanese has a structural disadvantage, as kanji --Han characters-- are much better suited as a writing system for languages that are similar to Chinese. Vietnamese had considerably less problems in adopting Chinese characters because it is an isolating tonal language like Chinese. Words in isolating languages are not subject to change and therefore don't need to represent aspect or time. In order to reflect morphological change, Japanese developed its own syllabaries. (kana) Hiragana is used for grammatical particles and inflectional affixes. The other set of syllabic signs, katakana, is employed to write non-Japanese loanwords and many adverbs. As a general guide kanji are used to write most nouns, verbs and adjective bases. When early in the first millennium AD Japan adopted kanji, at that time having no script of its own, two main strategies were employed to accommodate the script to the language. Kanji were selected on the basis of meaning and in relation to the sound. This process can be represented in the following way.

Meaning--3some, 3dent, 3iary, 3ce, 3d
Sound-----3angle, 3ceps, 3cycle

The reading of kanji that relate to Japanese morphemes (the smallest semantic elements in a language) on the basis of meaning is referred to as kun-reading. The kanji was assigned a Japanese pronunciation, sometimes more than one, and chosen for the correspondence with its meaning in Chinese. Although there can be multiple readings for one kanji, it also happens that some have no kun-reading at all.
The kun-reading of 流れ星 is naga(re)boshi, meteor, falling star. The on-reading or Sino-Japanese pronunciation is an approximation of the Chinese pronunciation at the time when the kanji was introduced. At different times kanji from various parts of China were adopted, with the result that some kanji have more than one on-reading and also have different meanings. There are three distinct echoes depending on whether the word was borrowed in the sixth century, the eighth century, or early in the millennium. The on-reading of 流星 is ryuusei, meteor, falling star. As many Chinese loanwords found their way into Japanese, it was only natural to write Chinese words with Chinese characters. This created an intriguing “distortion effect” as new words were created with kanji based on an approximation of the Chinese pronunciation combined with semantically selected kanji that ignored pronunciation altogether. Sino-Japanese morphemes like these have been instrumental in enriching Japanese with an extensive layer of vocabulary. This process has been compared to the role of Arabic within Persian and Turkish or Latin within the European languages. Sino-Japanese words relate to concepts that either were not in existence in Japanese at that time or were deemed to be more refined than their native equivalent. They take the form of kanji compounds (two or more kanji) that generally follow the on-reading. Single isolated kanji usually have a kun-reading.
There are according to tradition six types of kanji that, although no longer studied because of the confusing nature of the definitions, are useful in the sense that they shed light on the structure of kanji. The first four categories refer to structural composition, the last two to usage.

Shoukei-moji 象形文字 A fraction of modern characters consists of “pictographs”, characters that purpose to be a literal graphic representation of an object. Examples of this type include tree 木, or moon月
Shiji-moji 指事文字 Ideographs or logograms also represent a small number of modern kanji. These “symbols” essentially express a simple abstract concept like up and down 上下.
Kaii-moji 会意文字 A small number of modern kanji consists of “ideographs”, usually a combination of pictographs to present an overall meaning. An example is rest, made from tree 木 and person 人.
Keisei-moji 形声文字 The largest of the categories representing 85% of kanji, are the “semantic-phonetic” characters. Consisting of two components, the phonetic component refers to the (long obsolete) pronunciation of the character. In Japanese this is the on-reading. The semantic element indicates the meaning or context. In the passing of time the semantic context has frequently been subject to change, making the interpretation of a character in many cases rather confusing. Also, the distinction between phonetic and semantic elements of a character is ambiguous as phonetic elements frequently have semantic connotations as well. An example is mosquito (蚊) that combines insect 虫 with 文 BUN to give “insect that makes a BUN sound”.
Tenchuu-moji 転注文字 “Derivative characters” or characters with borrowed meaning and pronunciation is a rather vague category. As the majority of characters have undergone some change of meaning, this particular classification is not so relevant. Sen 占 used to mean divination but now has acquired a major meaning of occupy, replacing a more complex character with the same pronunciation.
Kasha-moji 仮借 文字 “Phonetic loan characters” are employed for their sound value to represent other but unrelated but homophonous morphemes. An example is lai “barley” in Chinese for lai “come” 来
Kokuji 国 字 “Made in Japan” characters are sometimes treated as a seventh category. The number in modern kanji is small. The most famous example is 働く hataraku that combines person” 人 radical with “movement” 動 to create “work”. The character has been adopted into written Chinese in the twentieth century.

Radicals

A radical in kanji first of all refers to the element that was chosen to classify the character for easy reference in a dictionary. 部首 (ぶしゅ) or “section head” contain characters with similar graphic components. Starting off initially with more than 500 radicals, the list was whittled down to 214 “indexing components” popularised later in a dictionary that became known as the Kangxi dictionary. (In Japanese known as the Kōki Jiten.) Radical is an unfortunate translation as the term is more appropriately applied to inflective languages which Chinese is not. In addition, radical refers to the “semantic component” in kanji as opposed to the phonetic component. The distinction between the two is not absolute as radicals sometimes also play a phonetic role and phonetic elements occasionally have semantic connotations as well.
Radicals within characters may appear in seven positions.

かんむり     top kanji radical                       (艹grass)    花 若 苦
へん             left-side radicals                       (土earth)    地 坂 城
つくり         right-side radicals                    (彡 three)   形 影 彫
たれ             radical top-left position           (广 cliff)     広 店 麻
あし             bottom kanji radical                 (儿 legs)     先 兄 光
にょう         radical bottom-left position    (辶 road)    近 道 送
かまえ         radical enclosed position        (門 gate)     聞 閉 開

Kanji Alchemy

Kanji Alchemy is a consistent and rigorous method for studying more than 6000 Japanese characters in order to facilitate reading Japanese texts. It is based on the grouping together of kanji that share common characteristics (clustering) and the selection of an individual character that is representative of that group (chunking). There is a further subdivision of these representative characters into seven different social areas that relate to significant human activities.
Clustering is based on the development or etymology of the character in question and this determines the allocation of kanji in the group.
The framework for allocating representative characters (chunking) corresponds to the pre-industrial belief-system of astrology. As the planets are associated with seven days of the week, each week constitutes one lesson and seven lessons represent one chapter. There are ten chapters in total that consist of seventy lessons. An alchemical symbol or glyph precedes each of the seventy weeks whereas each of the ten chapters is named after an astrological sign.
The rationale of Kanji Alchemy lies in the correspondences between the etymological background of the characters and the traditional spheres of planetary influences. In other words: kanji that refer to weapons are associated with Mars (the planet of war) whereas Venus (associated with love) is paired with characters that relate to sex. There are numerous kanji that point to religious ceremonies (Saturn, ruler of time/transition/death) and others that are based on trading (Mercury) or government (Jupiter). In addition, the Sun and Moon are broadly associated with masculine (Yang) and feminine (Ying) aspects such as the conventional dichotomy between heaven and earth, day and night and warmth and cold.
When studying one cluster of the 490 characters (7 days x 7 weeks = 49 x 10 chapters = 490) the following is an example to illustrate the method.

Tips for Finger Washing

先           セン、さき                 previous, precede, tip 先
指先       ゆびさき                     fingertip 3A
指           シ、ゆび、さ-す        finger, point 3A
洗          セン、あら-う             wash, investigate 洗
洗礼式  せんれいしき              baptism 3A
礼*         レイ、ライ                 courtesy, salute, bow 3A
式          シキ                             ceremony, form 3A

The alchemical symbol for the third week is Alum. The first day of the week (Saturn) features 先 as the signature character. Originally the character referred to “foot/stop + person, die, ancestors, precede, tip”. The mnemonic sentence is “Tips for Finger Washing” as there are two kanji in this cluster: 先 and 洗. As kanji compounds feature more often in texts than single individual characters, it will be more efficient to treat these as part of a combination of more than one character. Focusing on words rather than single characters facilitates recollection and makes more sense in the context of reading. Hence, rather than considering 先 and 洗 the emphasis should be on 指 先 and 洗 礼 式 . In addition to considering kanji as part of vocabulary it would also be more efficient to study these as members of a collection or cluster. The mnemonic sentence “Tips for Finger Washing” alludes to other kanji that are part of this group. Memorising the sentence will make it possible to systematically retrieve the other associated characters. The seven days of the week are in a fixed order starting with Saturn, followed by Jupiter, Mars, Sun, Venus, Mercury and Moon.