for有两种用法。
一是作介词，后面接名词、动名词、代词、一般疑问句和特殊疑问句作宾语，可表示目的或原因，例如：Researchers have proposed several explanations for why interruption of glucose processing might retard aging
二是作引导词，后面接陈述句，引导原因状因从句，表示“因为，原因在于”，例如：They are less fully given back to the water，for they still lay eggs on the beaches.
