Can you use が instead of は in sentences like this? 一番好きな食べ物がりんごです

Question

I am currently trying to understand the difference between は and が　in Japanese. As far as I have learnt, が　and は　can both be used after subjects, but は can shed a spotlight on the topic of the conversation. For practice, I constructed a few sentences of my own.

'ジョンくんはあまい食べ物が大好きです。一番好きなあまい食べ物がりんごです。'

In the second sentence, I still intend for the topic to be John, so I used が after 食べ物. However, I am unsure about whether this is correct because usually when I see sentences in this format, は is used instead of が.

Is my example correct or is there any language nuances that I have overlooked? Any help is deeply appreciated.

Answer 1

→ 一番好きなあまい食べ物 ← がりんごです
                ↑look here

the above が indicates the focus of the sentence is the subject clause. (don't look at the apple, that's not the new information). It's misdirection. は on the other hand, tells you that new information is coming immediately after: what's his/her favorite fruit? (IMPORTANT INFORMATION COMING): apples.

Answer 2

"か" proposes a new scene or topic. "は" maintains coherence and makes the article a whole. Your second sentence is also used to explain John's preferences. It should form a coherent semantics with the previous sentence, so "は" should be used.