There were no written scripts in Japanese, until Hanzi characters were introduced from China. These Chinese logograms became a part of the Japanese writing system, however words like prepositions, suffixes, prefixes and verb conjugations could not be represented by Kanji. Hence, the derivation of hiragana and katakana scripts followed.
Katakana is mainly used to write loanwords (gairaigo 外来語), but there are many other instances when Katakana script is preferred.
Let’s find out when and why Katakana script is used to write Japanese.
Loanwords or 外来語
According to the online Oxford Dictionary, a loanword is “a word adopted from a foreign language with little or no modification“.
Katakana is used when writing loanwords like スピーチ (speech)、トマト (tomato) or パーティー (party) in Japanese.
Some 外来語 gairaigo are unaltered, but some are slightly changed to suit Japanese pronunciation. For example, トマト to-ma-to is unaltered but サンキュー (thank you) is pronounced with ‘s’ but not ‘θ’. Because the sound ‘θ’ as in ‘thank you’ is not present in Japanese language.
Loanwords are also abbreviated, like ワープロ which means word processor. But instead of writing the two words, the first two syllables from both the words are combined to form one word which means the same thing.
Sometime words like 消しゴム keshigomu (eraser) and カラオケ karaoke（where カラ is 空）are formed by combining a Japanese word with a 外来語 gairaigo.
Jargons and Proper Nouns
Names of animals, plants or vegetables are often written in Katakana. Though ナス (eggplant) and サツマイモ (sweet potato) are not 外来語 (loanwords), you will find these names written in katakana when you visit a supermarket. Similarly, when you visit a safari park or zoo you will find names of animal species written in katakana, like サル(monkey) or トラ (tiger).
Giongo or 擬態語
擬態語 (gitaigo) are onomatopoeic words that imitate a particular sound or voice. For example, ザーザー(zaazaa) is a giongo that represents the sound of heavy rainfall, is written in katakana. Sounds of animals are also written in katakana, like モーモー (sound of cow), ニャーニャー (sound of cat).
Substitute for Kanji
There are two reasons for replacing kanji with katakana:
When using heteronyms – Kanji like 角 (corner) has two kunyomi 訓読み readings, かど and つの. In this case, 角 can be written in katakana as カド or ツノ, whichever pronunciation suits the sentence. 方（カタ or ホウ）and 空 (カラ or ソラ) are similar examples.
When using difficult kanji – Some kanji which have too many strokes are indeed difficult to remember and takes time to write. Hence, to ease the tension, words that are difficult to in write kanji, like 眼鏡 (メガネ) and 雲脂 (フケ) are written in katakana.
To put more emphasis
If you want to put more emphasis on a word or make it stand out in a sentence, use katakana. When exclamations like エーッ！or ホンマヤ！are written in katakana they appear more lively. Sometimes, 擬態語 gitaigo (mimetic words) are written in katakana to make them sound more rigid, like ピッタリ and ニコニコ.
Katakana is also used for writing negative words or slangs like ダメ and バカ.