1. when: 時を示す語が先行詞。
   The day when I must leave here is approaching.
   
   
2. where: 場所を示す語が先行詞。
   I will visit the village where I spent my childhood.
   
   
3. why: 理由を示す語が先行詞。
   Tell me the reason why she has got angry.
   
   
4. how: 方法を示す語が先行詞（通常は先行詞またはhowが省略される）。
   Do you know the way (how) he solved the problem?
   
   
5. that: whenとhowの代わりに用いることがある。where と why の代わりに用いることはまれである。
   His son was born in the year that he visited the country.

関係副詞の２つの用法

1. 限定的用法（制限的用法）：関係副詞が導く形容詞節が先行詞を修飾する。この用法の関係副詞は「前置詞＋関係代名詞」の形に書き換えることができる。
   
   Please let me know the exact date when he will return.
   Please let me know the exact date on which he will return.
   
   The reason why he had resigned remained unknown.
   The reason for which he had resigned remained unknown.
   
   
2. 非限定的用法（継続的用法）：関係副詞の前にコンマがある。この用法はwhenとwhereだけである。これらの関係副詞は「接続詞＋副詞」の形に書き換えることができる。
   
   He went to Europe, where he got married.
   He went to Europe, and there he got married.
   
   The ship drifted for a week, when the crew were rescued.
   The ship drifted for a week, and then the crew were rescued.

関係副詞または先行詞の省略

1. 限定的用法では関係副詞は常に省略できる。ただし、非限定的用法の関係副詞は省略することができない。
   
   The forest (where) I shot birds is seen over there.
   They lived in an age (when) airplanes were not invented.
   
   
2. 関係副詞の先行詞は常に省略することができる。先行詞が省略されると、関係副詞は名詞節または副詞節を導く。
   
   
   • 名詞節を導く場合：
     This is (the place) where he studied medical art.
     That is (the way) how I quarreled with him.
     
     
   • 副詞節を導く場合：
     He caught fishes where the water was shallow.
     Where there is a will, there is a way.
     Put it where you found it.
     
     
3. where が導く名詞節は、先行詞が省略されると、whereが疑問副詞と解釈できる。
   I do not know where he found it.
   
   
4. when は先行詞が省略されると、接続詞としての働きをする。
   He phoned to me (at the time) when I was out.