I'll help you with your homework as long as you promise to study hard.

as 在这里是连词，表示"在...的情况下"或"当...的时候"。 long 的本义是"长的"，在这里与 as 搭配形成固定用法。 as long as 是一个固定短语，表示"只要"的意思，用于引出条件。