for 和 because 作为连词都可表示原因。

because引导的分句为状语，所以可以位于主句前，也可以位于主句后。例如：

He feels very weak because he is ill.

Because he is ill, he feels very weak.

for作为并列连词，用来引出并列分句，解释说话人为什么会说前面的话。所以，for只能位于二个分句之间。例如：

They were certainly there, for I saw them.

不可以说：For I saw them, they were certainly there. (X)

并列连词and, but, so都可以位于句首，表示本句和前一句之间的逻辑联系。例如：

He sings well. And he dances well too.

He was ill. But he continued working.

He can keep it. Or he can give it to someone else.

for和其他并列连词一样，也可以这样用：

They were certainly there. For I saw them.

because则通常不可以这么用，除非是口语中回答“why".

They were certainly there. Because I saw them. (x)

because 可以替换 for, 但for未必能替换because。例如：

They were certainly there, for/because I saw them.

He sold his house, because he needed money for his son's operation. (不宜用for)

for=I say so because...