Cfrtjryk

1. Henry could not comprehend the message.
   
   


2. Henry could not understand the message.

Are those sentences the same in meaning perfectly?

Rarely are two synonyms "the same in meaning perfectly".

You've provided no additional context, so it will be natural for the reader to imagine some.

I can think of a few different ways a sentence like this might be used.

1. The message was sent in French, and Henry doesn't speak French. Therefore, Henry could not understand the message.




2. The message was sent of over the radio, but the reception was very poor, and the message was garbled. Therefore, Henny could not understand the message.




3. The message was about calculus, and Henry has never taken any math higher than geometry. Therefore, Henry could not understand the message.




4. The message was from Henry's lover, announcing that she wants to leave him. Henry thought their relationship was very solid; this pronouncement caught him totally by surprise. Therefore, Henry is having trouble comprehending the message.

I think both words could be used in all four contexts, but I still maintain there are some contexts where a writer might find one word might be a little bit more suitable than the other.

As a footnote, even though my answer differs from Lorel's answer, I'm still largely in agreement with that answer, too. As Lorel says, it takes a lot of work to "imagine some quirky nuance" where one word might work better than the other, and, as Lorel also states, "generally those two sentences mean exactly the same thing." But it's worth pointing out that there, depending on the context, one synonym might seem more fitting than the other, depending on why the message is so difficult to understand or comprehend.

They are the same.

Even if some people may imagine some personal quirky nuances to distinguish between the two words in meaning, generally those two sentences mean exactly the same thing.

From the Merriam-Webster definition of understand:

UNDERSTAND and COMPREHEND are very often interchangeable. UNDERSTAND may, however, stress the fact of having attained a firm mental grasp of something // orders that were fully understood and promptly obeyed // COMPREHEND may stress the process of coming to grips with something intellectually // I have trouble comprehending your reasons for doing this.

That is, both words mean "grasp the meaning of," but in some cases understand stresses the final result, while comprehend stresses the process of getting there. Most people use these words interchangeably, so this difference in stress isn't really apparent in isolated sentences like your examples, but in a larger context, choosing one word over the other could be appropriate.

For example:

Even though I tried to explain it to him for at least fifteen minutes in a dozen different ways, Henry could not comprehend the message. [stresses the process of trying and failing to comprehend]

Because Henry could not understand the message, he never returned my phone call. [stresses the result of the misunderstanding]