I have read that "understand" is a stative verb and means something like "to know well." However, lately I have had some doubts on whether that is always the case. For example, in the following sentence
"I can't understand what he's saying"
I'm not sure whether "understand" conveys an action or describes a state. If it was being a stative verb in that case, then substituting something like "know" should be possible. However, "I can't know what he's saying" sounds very wrong. In addition, dictionary definitions of "understand" often describe as "perceive; grasp the meaning of" which conflicts with its definition as a stative verb. Also, "comprehend" is often defined as "understand" even though I am fairly sure that is generally a dynamic verb.