When to use 'wa' and when to use 'ga'.
 

Is there anyone out there who can explain to me the differences in when to use 'wa' and when to use 'ga' as part of a sentence?

I understand that when asking a question it is usual to use 'ga' and then when replying to the question you would answer with 'ga'. But it seems there are many different rules to using these particles and it is very confusing!

Thank you!

oh no....
literally books have been written on this.
personally, I have given up on trying to understand this academically and simply try to"feel it out" most of the time. for me, most of the time, the difference mostly has to do with what you are placing emphasis on in the sentence...
but, be that as it may:
Japanese Particles - use of particle wa and ga
not a book, but pretty useful

"ga" is generally used when the topic is not fully known by the person being addressed, "onaka ga itai", or "my belly hurts". The other party wasn't aware your belly hurt before you mentioned it.

"Wa" is a topic marker, "ga" is a restrictive subject marker.

In Sangetsu's example above, the topic is you and the subject is your stomach. In a way, the sentence:

Onaka ga itai

is kind of short for:
Watashi wa onaka ga itai

Literally: I'm talking about me, and the tummy hurts.

(Sorry, not very helpful I guess)

Blimey, haha looks like this is a confusing one. :D

Thank you for the replies!