“I fail to do something” vs “I can't do something”

I am wondering why would someone say “I fail to understand” instead of simply saying “I can't understand”. Is there any specific difference between the two? Is it correct to say that the following two phrases are equivalent?

Excellent point, but I fail to understand how the example at the bottom is related.
Excellent point, but I can't understand how the example at the bottom is related.

If you fail to complete the form before departure, this may result in...
If you can't complete the form before departure, this may result in...

I am also wondering if “fail to do something” is equivalent to “can't seem to do something”.

Solution 1:

". . . I fail to understand . . ."

is an alternative to "I do not understand." The former could sound a bit snarky if said with a certain tone of voice that communicates "I think your example stinks!" or "I am not convinced your example is related" or "I think you haven't given me a good-enough reason to think your example is related."

When a person says "I can't understand," she more than likely means "I don't understand." Perhaps with further explanation, she'll come to understand. Can't could also indicate (by inference) she is really frustrated, with the voice inside her head saying "I can't do this, I can't do this, I can't do this!" In other words, she feels she cannot do this.

Your second set of examples can be improved, I think, by replacing fail to and can't with the word don't (or do not). Don't can be more neutral than fail, and hence can be more diplomatic and polite. Isn't it better to say

"If you don't complete the form by three o'clock, you'll be given another chance to complete it later"

than

"If you fail to complete the form by three o'clock, you'll be given another chance to complete it later"?

The latter example could sound a bit more judgmental than the former.

Even so, the word don't can imply a threat:

"If you don't clean your room, you're grounded, young man!"

Can't, as someone pointed out above, implies an inability to do something, for whatever reason (e.g., not enough knowledge, not enough time, a physical handicap, or whatever).

In a military context, however, the word fail can be quite appropriate. In that context, diplomacy and politeness are likely of no importance!

"If you fail to carry out this order, soldier, you will suffer the consequences!"

As to your last question, the two expressions are certainly similar, but they can be used quite differently. In the military example above, a soldier could fail to do something for a reason unrelated to inability--perhaps unwillingness. He just doesn't want to do it.

On the other hand, a person who "fails to do something" may fail because she is unable to do it, for whatever reason. Again, that reason could be insufficient training, insufficient skills (which may or may not be improved through practice), or even a basic unwillingness. A sin of commission, they say, is to do the wrong thing, whereas a sin of omission to fail to do the right thing.

It is not unusual for someone to talk herself out of doing something by saying

"I just know I can't do this!"

This self-defeating attitude is more common than you might think. Ask your grandmother, for example,

"Grandma, would you consider getting a computer so that we can email each other?"

To which she says,

"Oh, I just can't deal with computers! What's wrong with the telephone?"

Solution 2:

Fail to is more equivalent to don't.

Can't implies an inability to do something (e.g. the form is not available until after departure), rather than not doing it for some other reason (e.g. forgot to do it).

Solution 3:

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