Chinese vs Japanese
How to tell Chinese from Japanese
Chinese and Japanese are unrelated languages that share a writing tradition: Japanese borrowed Chinese characters (kanji) and a huge layer of Sino-Japanese vocabulary. Grammar, sound systems and politeness are completely different, and neither is intelligible to speakers of the other.
The short answer
Look for kana: rounded hiragana (の, は, を) or angular katakana mixed with kanji mean Japanese; a text made only of characters is Chinese. In speech, Chinese has tone contours and many -n and -ng endings, while Japanese has an even, syllable-timed rhythm with mostly vowel-final syllables.
The verdict
Kana (の, は, curvy hiragana, angular katakana) mixed in with characters means Japanese. Only characters, with 的, 了 or 是, means Chinese.
Closeness score
8 / 30
Mutually intelligible?
no
Shared vocabulary
About half of Japanese dictionary entries are Sino-Japanese (kango), many shared with Chinese in meaning
Which first?
Chinese first if you want both: kanji readings come easier.
Kungfucius says: Same characters, different language, and a toilet-paper misunderstanding waiting to happen.
Side by side
diànhuà電話
denwaA Meiji-era Japanese coinage that Chinese adopted; the characters match, the readings do not.
dàxué大学
daigakuJapanese writes the shinjitai form 大学, which matches Simplified Chinese here.
shǒuzhǐ手紙 = letter
tegamiLiterally "hand paper". In Japanese it means a letter; in Chinese it means toilet paper. Ask a Chinese friend to send you a 手紙 at your peril.
miǎnqiǎng勉強 = study
benkyōChinese 勉強 means doing something unwillingly or only just managing; Japanese 勉強 means studying (and, in shops, giving a discount).
deの
noBoth mark possession (我的書, 私の本), so they appear constantly and make a reliable giveaway.
àirén愛人 = lover, mistress
aijinIn mainland Mandarin 愛人 traditionally means spouse; in Japanese it means an extramarital lover. Taiwan Mandarin uses it as "lover" more broadly.
dàzhàngfu大丈夫 = all right, OK
daijōbuA Japanese learner's most-used word means "manly hero" to a Chinese reader.
qìchē汽車 = steam train
kishaSame compound, different vehicle: Chinese means a motor car, Japanese a steam locomotive train.