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 naturally say it in English (at least most people, I'd imagine), 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, it seems to be more on the "other part" (Japanese being understood, 3 children, the thing able to be done) for lack of a better term.

Answer 2

As for your question, both が and に are equally common for だれ{が・に}これが出来るか while が is more common for だれ{が・に}日本語が分からないか.

Because できる or わかる were originally intransitive verbs that meant 'appear' or 'split' respectively, they take a structure below.

私にこれができること = that this appears to me → that I can do this

私に日本語がわかること= that Japanese splits (itself) to me → that I can understand Japanese

(Since the likes of 私にこれが出来る is not a valid sentence without conditions, I use clauses instead)

However, as these verbs started to be used as a kind of transitive verbs, particle が started to be used as a marker for the agent of possible action. As a result, 私がこれができること or 私が日本語がわかること has been accepted.

When は is attatched to 私が, it changes to 私は and 私に changes to 私には. Through those manipulations, you can get sentences like 私はこれが出来る or 私にはこれが出来る.

Answer 3

There are a number of situations in which the subject can be followed by に.

1. When the predicate consists of potential verbs like わかる、読める、見える etc.
2. When the predicate consists of verbs or adjectives expressing a request/demand, such as 要る、欲しい.
3. When the predicate consists of adjectives expressing emotions/feelings, such as 嬉しい、悲しい、懐かしい.
4. When the predicate is a 敬語動詞.