English term (edited): ....

All of these imply someone who would like to think they have skills in some area and perhaps have had some training or experience, but in fact cannot perform at a high or professional level.

Not sure if those would fit your description, but I think that if the person hasn't mastered the skill in question yet, s/he can be considered an apprentice. Or an amateur.


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Note added at 3 hrs (2006-04-30 12:13:43 GMT)
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So, you are looking for a negative term?

Someone who is supposed to be able to do something but is incapable of doing it would be imcompetent.

:unable or failing to perform adequately

English term (edited): somebody who has practiced something many times but can't do it

we say someone is "hopeless" when they just can't seem to "get it" even though they are really trying (U.S.)

I don't think there is really an idiom for this
in fact, there is an idiom for the contrary

if at first you don't succeed, try, try again: Information From ...Idioms information about if at first you don't succeed, try, try again The American Heritage® Dictionary of Idioms by
www.answers.com/topic/ if-at-first-you-don-t-succeed-try-try-again

as a general term
all thumbs if it's a manual exercise
slow on the uptake if it's a mental exercise

English term (edited): if you mean ناشی

inapt/ inexpert/unskilled/unskillful

English term (edited): [s/o who is incompetent, despite having practised a lot]

As has already been said, I don't actually think we have a specific, parallel idiom in EN, but this single word comes close to capturing part of the spirit of it.

A bungler is certainly someone who is incompetent/inept; but it does convey a slight 'backpack' meaning of '...despite more than one attempt' at doing whatever --- an habitual failure. Goes a little way towards the meaning you're after...

and master of none

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Note added at 7 mins (2006-04-30 09:18:21 GMT)
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see this ref:

The "Morris Dictionary of Word and Phrase Origins" by William and Mary Morris (HarperCollins, New York, 1977, 1988) gives further insight into the phrase:
"jack of all trades, master of none" has been around at least since 1600. Anyhow, the 'jack of all trades' part of it was circulating at that time and, generally speaking, it was a term of praise, rather than disparagement, as it is today. One writer noted 'Old Lewis' was sort of 'Jack of all trades,' which made the rest of the tradesmen jealous.' 'Jack,' in those days was a generic term for 'man.' Later the 'master of none' was added and the expression ceased to be very flattering. Today it is used to describe a person whose knowledge, while covering a number of areas, is superficial in all of them. If you want a very elegant word to describe such people, call them 'sciolists' (pronounced SY-uh-lists). They won't know what you're saying and will probably take it as a compliment."

http://www.phrases.org.uk/bulletin_board/4/messages/936.html

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Note added at 21 hrs (2006-05-01 06:30:02 GMT)
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if it really is only about incompetence, then there is the expression for someone who is never good with their hands as being "all fingers and thumbs"

Fingers and thumbs - UsingEnglish.comIdiom Definition:. Meaning:. If you are all fingers and thumbs, you are being clumsy and not very skilled with your hands. ...
www.usingenglish.com/reference/ idioms/fingers+and+thumbs.html


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Note added at 21 hrs (2006-05-01 06:35:03 GMT)
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if it is about general ineptness, then

"he couldn't find his way out of a paper bag"

might fit the bill, or similar expressions at this ref:

He's one brick shy of a load. - WordReference ForumsHis elevator doesn't go all the way to the top. He couldn't find his way out of a paper bag. He doesn’t have both oars in the water. ...
forum.wordreference.com/showthread.php?t=96048

These are offered more as possibilities than definite answers. Knowing more of the context might help. A loser could be someone who no matter how hard he tries, practices, prepares, nevertheless fails. Thus, he could also be called a failure.

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Note added at 5 hrs (2006-04-30 14:29:03 GMT)
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Standard use: "What a loser; he can't do anything right." "I feel like a loser. No mater how hard I try, I can't do anything right." Does that help? It doesn't mean necessarily that the person has practiced or has much experience, but still fails, but it can apply to such a person, I believe.

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Note added at 1 day39 mins (2006-05-01 09:50:24 GMT)
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I wanted to add this up there under notes to/from asker, but I can't figure out how to do that. So, the pith is he's been doing something for a long time, but still doesn't get it, still doesn't understand the best way to do it, or how to accomplish his goal. He doesn't know which side of the bread is buttered? By the way, in pursuit of this, I found an interesting site:http://www.idiomsite.com/

Timeless or endless novice