1. because连词，意思是“因为”，表直接原因，语气最强，用来回答“why”开头的特殊疑问句；because引导原因状语从句，一般放在主句后。如：
(1) We stayed at home because it rained. 因为下雨我们呆在家里。
(2) They look small only because they are much farther away. 它们看起来小只是因为它们离我们很远。
2. as连词，意思是“因为”，语气比because弱一般用来说显而易见的原因和理由。如：
(1) As you were not there, I left a message. 因为你不在那里，我留了个信儿。
(2) As she is ill she will need some help. 她由于生病可能需要帮助。