Understand, comprehend, appreciate mean to have a clear idea or conception or full and exact knowledge of something.
Understand and comprehend both imply an obtaining of a mental grasp of something and in much of their use are freely interchangeable.
But understand may stress the fact of attained grasp, and comprehend may stress the process by which it is attained; thus, one understands a decision when he knows what it involves even though he fails to comprehend the reasoning process on which it was based; a person may understand a foreign language without comprehending exactly how he learned it.
Sometimes the difference is more subtle, for understand can imply the power to receive and register a clear and exact impression, and comprehend can imply the mental act of grasping clearly and fully; thus, the concept of infinity can be understood theoretically though scarcely comprehended as a verity.
Appreciate (see also APPRECIATE 2 ) implies a just judgment or the estimation of a thing’s true or exact value; therefore the word is used in reference to persons or things which may be misjudged (as by underestimating or overestimating or by undervaluing or overvaluing).