"Notice that the particles for the verbs in the たい form do not change in the sentence. However, the object marker を is [b]often replaced[/b] with が."
Since I am using Audio books, I have heard the たい-Form quite often in combination with を。 Actually more often than with が。 (本を買いたいです。 etc.) Since the grammar section says "often replaced" I was wondering when to use を and が properly. Right now it seems to be interchangeable.
Are there any tips on when to use the right particle?