两者均可表示“清晰地”、“不模糊地”，有时可换用，但是若其前有修饰语，则只能用clearly。如：

He spoke loud(ly) and clear(ly). 他说得既宏亮又清楚。

He spoke quite [very] clearly. 他说得很清楚。

I saw all that quite clearly. 这一切我都看得很清楚。

Speak clearly so that they may understand you. 说得慢一些以便他们能听懂你的话。