Somebody who pretends to understand, but doesn't [closed]
The person is a poseur (or poser) with respect to understanding.
Noun: poseur pow'zur
A person who habitually pretends to be something he is not
- poser
-- WordWeb
You can use Charlatan.
Its meaning in the Cambridge dictionary is
a person who pretends to have skills or knowledge that they do not have.
You should also check out its synonyms and related idioms at the above link to see which one better suits your purpose.
I would describe such a person as a bluffer.
This captures the intended meaning that the person is deliberately pretending to know more than they really do.
For example, they could bolster their act by consulting a "Bluffer's Guide" to the subject they are pretending to understand.
There are some good answers here. Another possibility is wiseacre which The Chambers Dictionary defines as follows:
wiseacre noun: someone who unduly assumes an air of superior wisdom; a wise guy; a simpleton quite unconscious of being one.
In ancient Greek theater, there were three stock characters in comedy: the Eirôn , the Alazôn, and the Bomolochus. Of those three characters, the alazôn is closest to the person whom you are describing in your question as pretentious, clueless, confused, and boastful (see here, here, and here).
The alazôn is the boastful imposter who, in essence, claims to know more than he does (which is very little). Along comes the eirôn who bursts the alazôn's bubble by pretending to know less than he does (which is a great deal), a process which could fairly be called a humorous Socratic dialog.
Traditionally, a Socratic dialog is a means of getting at the truth of a matter by question and answer, question and answer. In a Greek comedy, however, the humorous interaction between eirôn and alazôn proceeds step by step until the alazôn is exposed as a phony. In effect, the eirôn skillfully and humorously causes the alazôn to paint himself into a corner.
If you are thinking of a real-life character whom you know personally, you might try being an eirôn to his or her alazôn. Simply, but skillfully, ask the person a series of questions which is designed to make the person squirm, and possibly even confess to his ignorance. In other words, make him feel ashamed for being such a pretentious braggart!