が and に interchangeability and difference in meaning

Question

Consider these sentences:

• だれ{が・に}これが出来るか

• だれ{が・に}日本語が分からないか

When both が and に are acceptable, what is their difference in meaning and practical usage?

Answer 1

This is a great question, and one of which I'm not sure I fully understand the nuances. But here goes:

What I learned in my first Japanese class was the は／が for basic things like this:

• あの人は日本語がわかる　→　That guy understands Japanese.
• 友達は子供が３人います　→　My friend has 3 children.
• だれがこれが出来るか　→　Who can do this?

Then I heard some people start using に and I was like WTH? But after hearing に for a while, it seems to translate like "unto 〜" or "by 〜". It's not how we'd (at least most people, I'd imagine) naturally say it in English, so it seems a little strange.

• あの人に日本語がわかる　→　"Japanese is understood by that guy," or "Japanese is understandable unto him."
• 友達に子供が３人います　→　"3 children exist unto my friend," or "My friend has 3 children (unto himself)."
• だれにこれが出来るか？　→　For/To whom is it possible?

My understanding is that with the は／が the emphasis is more on the person/subject, whereas with the に version, itseems to be more on the "other part" (Japanese being understood, 3 children, the thing able to be done) for lack of a better term.