Difference between "as", "since", and "because" As we've known, "as", "since" and "because" are all used to express the reason. However, there certainly are some differences between them. What are they? And why we can always use "because" in place of "as", "since" while we cannot always use "as" and "since" in place of "because". Can anybody know the answer? Please help me. Thanks in advance.