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I found these sentences in a book:

> Kaigi wa itsu arimasu ka?  
>  Kore wa nan desu ka?

Could anyone please tell me why it's ..itsu desu.. and not ..itsu **ga**
desu..Similarly, why it's ..nan desu.. and not ..nan **ga** desu.., whereas in
these statements, 'ga' is used:

> Kono heya ni dare **ga** imasu ka?  
>  Ano heya ni nan **i** **ga** arimasu ka?

Also, is it wrong to say 'nan ga' in the second statement above?

I'm pretty new to Japanese grammar and like many others, I'm not very clear
regarding the usage of 'ga'. From the websites I've gone through, I've come to
assume that one of the uses of 'ga' is to indicate a specific person, thing,
etc and so when it's used in simple sentences, it follows interrogative words
like 'dore', 'dochira', etc.

For example,

> Dore **ga** anata no hon desu ka?
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questions particle-が
When to use the particle 'ga' in questions?
As you said, "ga" is used to indicate specific things, and in some cases can
serve as a topic marker. In the first two examples your topic is being marked
by "wa". In the case of the latter three examples, the topic.

The first example asks " **When** is the meeting?".  
Kaigi (meeting)  
wa (topic marker)  
itsu (when, question word)  
desu (copula, in this case "is")  
ka (question marker)

In this example you are talking about when _the meeting_ is, and "when" is
serving more as a property of the meeting.

An example of what I mean is "Watashi wa juu ni ji ni kaigi ni ikimasu.",
translated as "I will go to the meeting at 12 o'clock."

In this case the first "ni" marks the specific time, "12 o'clock", and the
second "ni" indicates the location I will be going to ("ikimasu" being the
-masu form of iku, or "to go"). "Wa" marks the topic, myself.

The question form would be "Watashi wa itsu kaigi ni ikimasu ka?", which asks
"When will I go to the meeting?". The topic is still myself, and in this case
you do not need a "ni" particle for the time ("when"), though you still need
to mark your destination ("kaigi") with a "ni" particle.

Going back to the example, an answer to the question would be "Kaigi wa juu ni
ji han desu.", or "The meeting is at 12:30."

In this case there is no particle marking the time because the topic is the
meeting, and the time serves as a description of the meeting (being
marked/followed by "desu"). It would be wrong in the question form to say
"itsu ga", because "when", as I said, is serving as a description of the
topic, not as the topic itself.

The second example follows a similar pattern. An answer to " **Kore** wa nan
desu ka?" would be "Kore wa pen desu." or "This is a pen." "Kore" ("this") is
the topic and "pen" is describing it. "Kore wa nan ga desu ka?" would also be
wrong because again, "kore" is the topic, and "nan" is a description of the
topic.

The last three examples you provided however are different, specifically in
that the question word _is_ the topic.

The third example asks " **Who** is in this room?". As you saw in my example
of when I will be going to the meeting, the location "kono heya" is marked
with "ni", since it is the location of the topic "dare". "imasu ka?" is asking
"what animate/living object is at the location?" In an example answer
sentence, "Kono heya ni sensei ga imasu.", would say that "the teacher" or
"our teacher" (remember, context can be crucial for small nuances in any
language), is in "this room". In this case, "wa" is not used since the topic
is followed by "imasu" (-masu form of "iru").

The fourth example is asking " **What** is in this room?", and is just two
words different than the third. "Nani" is "what", and "arimasu" (-masu form of
"aru") is basically the same thing as "imasu", except it is used for
inanimate/non-living objects. You wouldn't say "dare ga arimasu ka?" instead
of "dare ga imasu ka?", just like you wouldn't say "what is there?" instead of
"who is there?" in English.

Another ordering with (I think, someone correct me if I'm wrong:) the same
exact meaning, would be "Dare ga kono heya ni imasu ka?" and "Nani ga kono
heya ni arimasu ka?".

The fifth example asks " **Which** is your book?"  
"Dore", which "ga", marking the topic, asking _specifically **which_** item it
is "anata no", your "hon", book "desu", copula "ka", question marker

Again, "your book" is describing the topic, which in this case happens to be
unknown. An answer would be "Kore ga watashi no hon desu." or "This is my
book.", and this is where the extended meaning comes in. When you say "ga" in
the answer here you are saying "this is my book" and are implying "and only
this book is my book". If you say "Kore wa watashi no hon desu." you are
saying still that the book you are indicating/holding is "my book", but you
are not saying that for sure that is the only book that is yours. Using "ga"
would be like emphasizing "This" in " _This_ is my book.", or saying "This,
and only this book, is my book." Using "wa" would be like saying "This is my
book [but so is that one that you're holding, and that one over there]." (<-
implication in []'s).

To put it simply, "wa" and "ga" can both serve as topic markers, with "ga"
being more explicit and specific. In addition, when the topic is also the
"unknown" part of your question, you need "ga" because you're asking
specifically for an answer.

To my knowledge you can't, or it would be extremely unnatural to say for
example "Dore wa anata no hon desu ka?" (if I'm wrong/not completely right can
somebody more experienced correct me?), and I can't think of any example where
the topic would be unknown that you would _not_ use "ga" to indicate it. As I
said earlier, in the first two examples the unknown "variable" so to speak in
the sentence is not the topic, but something that _describes_ the topic, and
is therefore not marked with "ga".

An example of where you would use "itsu ga":  
"Itsu ga ii desu ka?", in English "When [specifically] is a good time [for
you]?", or colloquially "When is a good time?".

"Nan" and "nani" are both what and indicated by the same kanji " **何{なに｝** ",
so your fourth example already covers an example of where "何が{なにが｝" would be
used (I cannot however, thing of a place where you would say " **nan** ga"
instead of " **nani** ga"), but as another example of where "何が{なにが｝" would be
used is "Nani **ga** John-san no suki na tabemono desu ka?", which is in
English. "What is John's favorite food?".

"Nani ga", what specifically is  
"John-san no", John's  
"suki na", literally "liked", colloquially in English we would say "favorite"  
"tabemono", food "desu", copula "ka", question marker

I hoped this answered your question. If my answer didn't clear up your
confusion please comment so I can improve my answer for you.
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answer
## Asking and Answering Questions

The use of the particle が{ga} here doesn't change whether it is used in a
question or a statement (such as an answer). For example, a statement with the
"copula" です{desu} (the verb _to be_ ) and a question differs only by the
ending with "か{ka}?" to mark questions. Note that "か{ka}" is only used for
questions in formal language (with verbs ending in ~ます{masu}) and informal
language uses a rising tone.

Kaigi wa itsu arimasu ka?

Kono heya ni dare ga imasu ka?

> これは **何{なん}** です **か** ？
>
> これ{kore}は{wa} **なん{nan}** です{desu} **か{ka}** ？
>
> What is this?

An answer differs only by omission of "か{ka}?"

> これ{kore}は{wa} **ペン{pen}** です{desu}。
>
> This is a pen.

## The verbs い{i}ま{ma}す{su} and あ{a}り{ri}ま{ma}す{su} for existence

This similarity between statements and questions also occurs with phrases
using the verb _exist_ (which is often used for "there is" or "to have" in
English) which is い{i}ま{ma}す{su} for animate objects (people and animals) and
あ{a}り{ri}ま{ma}す{su} for inanimate objects.

この部屋{へや}に誰{だれ}がいますか？

> この{kono}へや{heya}に{ni} **だれ{dare}** が{ga}い{i}ま{ma}す{su} **か{ka}** ？
>
> Who is in this room?

The same syntax applies to the answer with か{ka} omitted:

> この部屋{へや}に **トムさん** がいます。
>
> この{kono}へや{heya}に{ni} **トム{tomu}さん{san}** が{ga}い{i}ま{ma}す{su}。
>
> Mr Tom is in the room.

Note that Japanese people affix ~さん{san} when referring to other people. Never
use あ{a}り{ri}ま{ma}す{su} (or the counters ひ{hi}と{to}つ{tsu}, ふ{fu}た{ta}つ{tsu},
etc) for people. It is considered very rude (not just a grammatical mistake).
However the syntax for objects is very similar:

> あの部屋{へや}に **何{なに}** がありますか？
>
> あの{ano}へや{heya}に{ni} **なに{nani}** が{ga}あ{a}り{ri}ま{ma}す{su} **か{ka}** ？
>
> What (inanimate object) is in that room (over there)?

The use of the particle が{ga} here is due to the use of phrases with verbs
with a subject rather than whether they are combined with a particular
question word. However, particles may be dropped when they are obvious and
would not impede meaning. This is most common in conversational language with
the particles は{wa}, を{wo}, に{ni}, and の{no}.

## Readings of 何

なん{nan} and なに{nani} do not have different meanings: they are different
readings of the same Kanji (何) meaning "What". There's no really a well
defined rule to determine which reading to use but it does seem to depend on
which sound follows it in the sentence (and is more "natural" for native
speakers to pronounce).
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