The city of New York ( the city = New York)

[sky3120]

a) used to state specifically which thing of the general type mentioned you are referring to:
the city of New York
the art of painting
the problem of unemployment




The city= New York

The art = painting

The problem = unemployment


Then, why is "the word of understand" and "the expression of "I got it" wrong?

The word = understand (?)

The expression = I got it (?)

However, I respect your words and I think you all are right but I would like to get out of this long time misunderstood issue this time.

Please help me out. Thank you.

[Tdol]

Then, why is "the word of understand" and "the expression of "I got it" wrong?

Are these grammatically the same as your examples? Understand is a verb and the second is a direct quote and not an example of anything.

[Chicken Sandwich]

sky3120 is referring to a comment made by Rover_KE:

Note that 'of' in your title is redundant. You have recently created other titles in that incorrect form - such as The word of understand instead of The word 'understand'.

Rover

[Rover_KE]

To be honest, sky, I was hoping somebody else would help me out here.

I can understand your confusion, and don't really have a good explanation for it, except to say it's a matter of usage.

When we are quoting a word, phrase, expression etc, we use inverted commas and no 'of' - like this:

The proverb 'a rolling stone gathers no moss' is often misused.

The word 'gay' used to mean 'happy'.

Rover

[Raymott]

I don't have the answer, but I'll make a few points.
Some words have two meanings, eg. "expression" can refer to the actual words, or to the saying or expressing of those words. So, "the expression X" refers to that expression, while "the expression of X" refers to expressing or saying 'X'.
"The translation of 'X'" means a different thing to "the translation 'X'." In the first, you're taking about X's translation, and in the second you're talking about X itself.

In your second examples, you mean "the word understand", "the expression I got it wrong", you're referring to something quoted. You don't need 'of' if the word or expression is the following word or expression.