differences

When I hear these two said by a person the distinctions I make is that "Fail" implies an honest attempt was initiated with the reasonable expectation that one would not fail at it.

whereas "Can't" implies that the action is judged to be beyond the person's capability after weighing the situation and deciding that it cannot be accomplished, probably before even applying any effort.

taking this to your specific statements it still holds true although in your example we are modifying the circumstances surrounding the accomplishment or the task of Cognisance or "understanding."

In one case it points to the Task as being out of reach in general. where the circumstances are the limiting factor (time, or environment, or materials) where the other one points at the person speaking as offering their own cognitive limitations as the factor of the disparity.

Normally when I hear a person use the phrase,

I can't understand, _____

it is usually followed by an adverb, like why, how, what and then a condition which is contrary to the LOGIC that the speaker expects most people; or the "Royal They," those damned decision makers, to have.

Frequently this is a rhetorical question meant solely to highlight some contraption or condition with which the speaker disagrees.

I can't understand why jessica is still dating that schmuck after all the times he's cheated on her.

I can't understand why anyone would put banana slices on a pizza?!

I don't often hear someone use I can't in complex sentences where it means the same thing as: "I am unable to achieve that"

More often in this case, I hear the specific reason, like,

I don't have enough time. I don't have enough money I don't have the tools It's 4pm in Los Angeles.

"Can't" seems to direct the fault of the unworkable dilemma to conditions which the responsible party wouldn't be reasonably expected to change or overcome. If the inability to achieve the condition has to do with external factors you would only hear someone use the word "fail" if they were being sarcastic about taking responsibility.

Fail points at the person in the statement as the limiting factor where one might normally expect the results to be something other than failure; however, "I fail to understand@ s one of those phrases which can be used to criticize the intelligence of the other party, said in a haughty voice, maybe even a little oxford in the accent, it's more of a condescending statement which calls into judgement the soundness of the person who presented the absurd concept in the first place.

"I fail to see the relevance in your argument sir. Maybe you can connect the dots for those of us who have difficulty following your meandering Fili-Bluster."

I think that

fail to understand takes responsibility.

can't Understand places it on the condition.

hope that helps

What I can't understand is why the heck I spent an hour writing this out when this post is from 2013...

David Patrone napkindiaries