words - 水 - pronounce choice between 水{すい} and 水{みず}

As the title says, 水 has 2 pronounce: 水{すい} and 水{みず}.

My question is:

• When to use which? e.g How to read it in: 水と米.


• What is the rule, if any.

Answer

Kanji have two sorts of readings: ON readings (音読み) and kun readings (訓読み).

Material that purports to teach you kanji usually indicates ON readings by typesetting them in katakana or (if in romaji) with capital letters. Kun readings are indicated by hiragana.

The thumb rules are:

1. 
   

   If a kanji has okurigana (e.g. 食べる, read た + べる), it is read using its kun reading.

   
   
   


2. 
   

   If a kanji is alone, with no matching okurigana, it is read using its kun reading.

   
   


3. 
   

   If a kanji is in a compound with a second kanji (水泳、海水 etc), it is read using its ON reading.

   
   


4. 
   

   Kun readings sometimes go through voicing (濁り) when used as the latter part in a compound word. Example: ゴミ箱{ばこ} from ゴミ + 箱{はこ}

   
   



5. 
   

   There are loads of exceptions to these above rules and your best bet is consulting a dictionary and making educated guesses. Compounds formed from ON+ON, kun+kun, ON+kun, kun+ON are all possible.

   
   

Your teaching material should have taught you these rules already. If it hasn't, it's not very good material.

水 has the readings みず and スイ

水 is read みず in your 水と米 example (rule 2)

水 when alone is read スイ when it means 'Wednesday' or some other assorted rarely-encountered things. (Check your dictionary.)

水 is read スイ when it is used in the compound 水泳{スイエイ} (rule 3)