as作因为时的用法，和because,since,for,有什么?
as 在意义上=because,一般来说because 不用再句首，要用句首就用as since之意，其实是既然这样，也可做因为解释，但有既然之意 for 与其最弱，后跟前面说话的根据 he won't come here,for i have seen that he was on the way home