今回は接続詞 as soon as について説明していきます。as soon as S + V 〜は「〜すぐに」を意味します。
 
As soon as the thief saw the policeman, he ran away.
「警官を見るとすぐに、泥棒は逃げた」
 
「〜すぐに」を意味する接続詞は as soon as 以外にも the moment S + V 〜・the instant S + V 〜・the minute S + V 〜 があります。
 
As soon as the thief saw the policeman, he ran away.
= The moment the thief saw the policeman, he ran away.
= The instant the thief saw the policeman, he ran away.
= The minute the thief saw the policeman, he ran away.
 
また、「〜すぐに」は否定の副詞 hardly・scarcely・no sooner を使って表すこともできます。
 
「〜すぐに…した」
Hardly had S done 〜 when［before］ S + 過去形 …
Scarcely had S done 〜 when［before］ S + 過去形 …
No sooner had S done 〜 than S + 過去形 …
 
Hardly・Scarcely・No sooner の直後は had S done 〜 と 疑問文の語順 になることにも注意しましょう。
 
「警官を見るとすぐに、泥棒は逃げた」
Hardly had the thief seen the policeman when he ran away.
Scarcely had the thief seen the policeman when he ran away.
No sooner had the thief seen the policeman than he ran away.

＜その他の例文＞
As soon as she closed her eyes, the phone rang.
「彼女は目を閉じるとすぐに、電話が鳴った」
 
The moment I left the house, it started raining.
「私が家を出るとすぐに、雨が降り出した」

＜補足＞
Hardly had S done 〜 when［before］ S + 過去形 … は、文頭に否定の副詞が移動したことで、倒置が起こった語順になっています。この形をふつうの語順に戻すと、hardly は S had hardly done と not と同じ位置 に戻ります。これは Scarcely・No sooner でも同じ現象が起きます。
 
Hardly had the thief seen the policeman when he ran away.
= The thief had hardly seen the policeman when he ran away.
 
Scarcely had the thief seen the policeman when he ran away.
= The thief had scarcely seen the policeman when he ran away.
 
No sooner had the thief seen the policeman than he ran away.
= The thief had no sooner seen the policeman when he ran away.

＜まとめ＞
・as soon as は 接続詞く
・as soon as は「〜すぐに」を意味する
・as soon as は the moment・the instant・the minute でも表せる