When it's OK to omit "where?" For example, is it OK to omit it in the following sentence?
This is the one of the few places (where) you can breathe real air.
In which situations one's not allowed to do this?
When it's OK to omit "where?" For example, is it OK to omit it in the following sentence?
This is the one of the few places (where) you can breathe real air.
In which situations one's not allowed to do this?
You can generally omit where when the head noun suggests a place:
1a. This is the one of the few places [ where you can breathe real air. ]
1b. This is the one of the few places [ you can breathe real air. ]
Here, the head noun is place, which of course suggests a place. However:
2a. This is the web page [ where the claim was first made. ]
2b. *This is the web page [ the claim was first made. ]
Example 2b is ungrammatical because web page is unlikely to suggest a place.
(Examples 2a and 2b are taken from "A Student's Introduction to English Grammar" by Huddleston and Pullum, page 185.)
In this answer, * marks a sentence as ungrammatical in standard English.
When the word where changes the meaning of the sentence you are not allowed to omit it.
For example:
In the goldmine, where the dwarfs work, the air is unbearable.
where refers to the goldmine. But when you remove it, the sentence is no longer correct.
*In the goldmine, the dwarfs work, the air is unbearable.
You would have to change it to:
The dwarfs work in the goldmine. The air is unbearable.
And that still doesn't completely mean what the first sentence means.
The second would be questions indicating location.
Where is ...
This is basically a special feature of the noun place. For many other nouns that have similar meanings, it's not possible to omit where.
I posted an answer to another question that gives more details: Relative pronouns "where" and "when": where can they be omitted?