は/が with いる. When to use each of them

Question

My textbook says the correct structure is

WHEREにWHOがいる

For example:

日本にかおるさんがいる

But I've seen some webpages with examples such as:

かおるさんは日本にいる

So... are they both correct? Do they have different meanings? Is one of them wrong?

Answer 1

Both are correct but their meaning isn't exactly the same.

An easy way to understand it is to think of them as answers to different kind of question :

庭に犬がいる。

(There is) a dog in the garden.

Can be the answer to :

庭に何がいる？

What's in the garden?

Whereas :

犬は庭にいる。

The dog is in the garden.

Can be the answer to :

犬はどこにいる？

Where is the dog?

In the former example, the word precedeing が is the new information (which in this case can be translated with the indefinite article "a"), the hearer and the speaker both know which garden they are talking about.

EDIT : This distinction between known and unknown information is probably 100% true when　に is followed by は (庭には...) but 庭に犬がいる can also mean "there is a dog in a garden" with both informations being new to the hearer.

In the latter example, the word preceding は is the known information (translated with the definite article "the" here, they know which dog they are talking about), and the word preceding に is the new one.

So in your case :

（誰が日本にいる？）日本にかおるさんがいる。　

(Who is in Japan?) (It is) Kaoru (that) is in Japan.

VS

（かおるさんはどこにいる？）かおるさんは日本にいる。

(Where is Kaoru?) Kaoru is in Japan.

You can find a similar construction with だ :

僕は学生だ。 (What are you?) I'm a student.

VS

僕が学生だ。 (Who is the student?) I'm the student.