Hello everyone,

I am confused about the usage of "of". Is there a quick tutorial that I can read about it? For example

could you tell me if there's a difference between these following sentences?

1) Department of Domestic Security

2) Domestic Security Department


3) The language of conversation

4) The conversation language

how about this?

5) No matter you level of proficiency ...

6) No matter you profeciency level ...

So, is there a rule?

Thanks in advance.
I don't know of a rule, and I don't know of a tutorial.

Often the choice between such phrases is made on the basis of how it sounds within the sentence where it occurs. It is often a matter of style and context whether you use, for example, level of proficiency or proficiency level.

There is no essential difference.

proficiency level......... Adjective+noun

level of proficiency ....... noun + prepositional phrase
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 CalifJim's reply was promoted to an answer.

There is no hard rule. All of the examples you provided are good grammar (in the US).

1. Both forms are commonly used for departments, for example, with no obvious grammatical change to the nouns.

- US Commerce Department; US Department of Commerce

In other cases, the use of an adjective is obvious.

- The German Chancellor; the Chancellor of Germany

2. "Of" tends to be used instead of "'s" in more formal writing and speech. This often leads to more inefficient, more confusing and longer documents. As a general rule, I go through my writing and eliminate "of" where possible.

- The most important of the impacts of the trade policy include....

- The trade policy's most important impacts include....

3. Use of "of" is often idiomatic, following tradition, rather than grammar rules.

- "the language of love" NOT "the love language" or "love's language"

- "at the end of the day" NOT "at day's end"

"Of" is one of the hardest English topics to master. Good luck....