Real subject vs Empty subject

[Van Khanh]

Hi,

In the sentence:

It is difficult to learn English

as well as:

To learn English is difficult.

"It" is an empty subject and "learn English" is a real subject. The these 2 sentences have the same meaning and are both correct.

But, when reading:

This word is used with singular countable nouns to refer to one of a number of things or people, when which one does not matter. (a)

Many people said to me that this sentence is totally incorrect even though the initial sentence with the empty subject is correct:

This word is used with singular countable nouns to refer to one of a number of things or people, when it does not matter which one.

Do you know why (a)(with real subject) is incorrect?

The same question with:

One or more of a number of people or things , especially when it does not matter which (one or ones) .

One or more of a number of people or things , especially when which (one or ones) does not matter . (b)

(b) is correct?

Thanks
K