On'yomi and kun'yomi: a kanji's two readings
After the kana, here's the next wall: one kanji can read two completely unrelated ways. Why does 水 (みず) sometimes read すい? The answer comes down to two families of readings, and one rule that bails you out most of the time.
Where the two readings come from
When Japan imported Chinese writing, it imported two things at once: the character and its rough Chinese pronunciation. That borrowed pronunciation is the on'yomi (音読み, おんよみ, "sound reading"). But the Japanese already had their own words for "mountain", "water", "eat": so they also pinned each character onto the existing Japanese word. That native word is the kun'yomi (訓読み, くんよみ, "meaning reading").
音読み (おんよみ)
The Chinese-derived reading. Often short and clipped (すい, さん, せい). Dominates when the kanji is glued to another kanji.
訓読み (くんよみ)
The native Japanese word mapped onto the character. Often longer, frequently followed by hiragana. Dominates when the kanji stands alone.
The rule that works (almost always)
Kanji alone or followed by hiragana (okurigana) → usually kun'yomi. Kanji glued to another kanji in a compound word (熟語, じゅくご) → usually on'yomi. It's not an iron law, but it gives you the right reading in the vast majority of cases.
水を飲む。
みず を のむ。
Drink water.
水 on its own, followed by a verb: kun'yomi reading みず.
水曜日
すいようび
Wednesday (literally "water day").
The same 水, glued to other kanji: on'yomi reading すい. Same character, opposite reading.
Why some kanji have several on'yomi
Japan didn't borrow Chinese in one go, but in waves, across several centuries and from different regions. The result: a single kanji can carry several on'yomi readings depending on when it was borrowed. The three main ones are 呉音 (ごおん, the oldest), 漢音 (かんおん, Tang era, the majority today) and 唐音 (とうおん, later borrowings).
The extreme case: 生
The all-time champion is 生 (せい / しょう / なま / き depending on context). It stacks several on'yomi readings (せい, しょう) and many kun'yomi: なま (raw, fresh), き (pure), いきる (to live), うまれる (to be born). Don't panic: you never learn readings in a vacuum, you meet them word by word, and context almost always settles it.