[Sahaqiel]

OK, so I looked it up in my book, which has been as loyally reliable as a book can be, and it says it refers to a reference to a general topic.

For instance,

私はりんごオ食べたいです。
I want to eat the apple.

私がりんごオ食べたいです。
I want to eat an apple.

Or something like that. (Unless I misinterpreted what my book was trying to say.)

Now, my Japanese Conversation teacher tells me it is the opposite.
I am getting a bit confused, here.
Incidentally, where is the object-identifying 'o' when using Japanese on your computer? Is this something I should know about?