The single most-misunderstood language pair. They look related - the writing overlaps heavily - but Japanese and Chinese are not genetically related at all. Different families, different grammars, different mouths.
What is actually shared
Characters: Japanese kanji are Chinese characters, imported from the 5th century on - 学生， 时间， 経済 mean the same thing in both. But watch the false friends: 手纸 is "letter" in Japanese, "toilet paper" in Chinese; 勉强 is "study" there, "reluctant" here.
Vocabulary: roughly half of Japanese's dictionary is Sino-Japanese loanwords - but pronounced in Japanese patterns (on'yomi), so 学生 is xuéshēng in Mandarin, gakusei in Japanese. Recognition transfers; sound does not.
What is not shared
- Grammar: Japanese is SOV with particles, honorific levels and agglutinative verbs; Chinese is SVO, uninflected, isolating. Nothing in Chinese grammar prepares you for Japanese grammar or vice versa.
- Sound: Mandarin is tonal with a complex syllable inventory; Japanese is pitch-accented with simple CV syllables.
- Writing mix: Japanese interleaves kanji with two syllabaries (hiragana/katakana) - Chinese is hanzi throughout.
Which to learn first
Learn whichever life points at. If both: Chinese first - its grammar is simpler, the character knowledge transfers wholesale into Japanese kanji study, and the reverse path buys less.
Common questions
Are Chinese and Japanese related?
No - unrelated language families. Japanese borrowed Chinese characters and vocabulary wholesale from the 5th century, but the languages' grammar and native vocabulary are different throughout.
Do kanji and hanzi mean the same thing?
Mostly yes for shared Sino-vocabulary (学生, 时间), but with famous false friends: 手纸 = letter in Japanese, toilet paper in Chinese; 勉强 = study vs reluctant.
Is Chinese or Japanese easier to learn?
Chinese grammar is far simpler; its phonology (tones) and writing are the hard parts. Japanese is phonetically easy but grammatically heavy. FSI rates Mandarin ~2,200 hours, Japanese ~2,200 too - different mountains.