Particle は replacing を - where does the stress lie?

Question

I've recently learned that the particle を can be replaced by either particles は/が. However, the stress on the focus of the sentence doesn't make sense to me.

My book gives this example:

ご飯を食べました。 (I've eaten DINNER; focus is on the fact that dinner was eaten)
ご飯はたべました。 (I've EATEN dinner; focus is on the fact that it was dinner that was eaten)

Shouldn't it be the other way around? From what I understand of は/が, the preceding word is set as the topic/focus of the sentence. So concerning the sentence ご飯はたべました。, the focus would be on dinner and not that they had already eaten dinner right?

Answer 1

First, が is a subject marker, を is an object marker. One cannot replace one with the other but, if you changed the tense from active to passive, the particle used would appear to change from を to が　 ie　from your first example to ご飯がたべられました. (Although it does not feel very common way to describe dinner.)

Actually I would say that the first sentence is not really placing strong emphasis on the object or the verb but definitely when you replace を　with は you are making dinner the topic and the emphasis is what comes after は. Perhaps the following is more natural:

「ご飯は、もう食べた」

"As for dinner, we already have eaten."

FWIW　が　can be used to place emphasis on the subject that comes before it so if you want to stress that dinner has been eaten you might say ご飯がたべられました (Although as indicated above, I can't imagine many occasions when one would say this)

The change in emphasis between は＆が is covered better in the second to last comment on the answers to this question: What is the difference between でなくand ではなく？