Using に with adjectives

Question

I want to know if there is a rule or way of thinking that makes it easier to understand which adjectives can be used in the following pattern.

体にいいです。
健康にわるいです。
酒によわいです。
電車に詳しいです。
その場にふさわしかった。

I tried using 子供に怖い and it was unacceptable, so I wish to know, what is the general relationship between the noun and the adjective and how do you know in what sense to use them and which adjectives it works with.

Can you explain using more examples? Please be as verbose as possible as I cannot find ANYTHING about it on the internet or in any textbooks despite copious amounts of searching.

Random ideas: Is it related to:

1. お金がないのに・・・
2. おみやげにチョコをもらった。

Answer 1

Because 体にいい is 体に(対して)いい, while 子供に怖い is 子供に(とって)怖い. It's a completely different grammatical form, with a different "subject". In one case, the subject is the-thing-that-is-defined-by-the-context-but-isn't-in-the-examples, and in the last case, the subject is the kids.

その野菜は体にいい。

These vegetables are good for the body.

子供にはその映画が怖い。

When you are a kid, this movie scares you. (Not "this movie is scary for kids", that's the point).

Answer 2

This に is the one meaning "for" or "(un)to". (Related: が and に interchangeability and difference in meaning).

• 体にいいです。　→　Good for your body = "healthy/healthful"
• 健康にわるいです。　→　Bad for your health
• 酒によわいです。　→　Weak (un)to alcohol = can't hold your liquor
• 電車に詳しいです。　→　Detailed/familiar with trains = knows a lot about trains
• その場にふさわしかった。　→　Was appropriate to/for that place

I don't know why 子供に怖い would be unacceptable if you're talking about something that is "scary for/to children". Unless you were trying to say "(I'm) afraid of children", then that's not correct. If 子供に怖い is indeed unacceptable, I'll let someone else chime in on that because it sounds fine to me.