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?