Difference Between 手伝う, 力になる, and 役に立つ

Question

As the title says. I'm playing the Japanese version of a game and came across the last two words. I've researched into them and the best I can describe them is that they both mean "to be helpful/useful."

In my notes about vocab I come across in the game, I write that the difference between 手伝う and 力になる is that the latter is used for, and I quote, "slightly bigger situations or wanting to be 'dependable' in some way." However, I'm unable to discern the difference 力になる and 役に立つ.

So, my main question is, what are/are there any fundamental differences between these three words and, if so, what context is each appropriate to use in?

Answer 1

Grasping and describing the exact meaning of words is an arduous task, and these pretty similar words probably differ in scores of subtle ways I can't even fathom, but here are a few notable differences between them I can see and put into words.

Crudely put, "手伝う" means "to help" (in the sense of "lending a hand") while "役に立つ" means "to be useful/helpful". "手伝う" often implies being useful, but not necessarily. For example, you may say "父の仕事を手伝ったが、あまり役に立たなかった。".

"役に立つ" usually refers to affording practical help, while "力になる" is often about lending moral support (though it can be used about practical help, too). You might hear an Olympic athlete, interviewed after winning a gold medal, say something along the lines of "皆さんの応援が力になりました。", but probably never "皆さんの応援が役に立ちました。".

In terms of absolute frequency, I suspect "力になる" comes in dead last. (For reference, there are 220 raw hits for "力になる" on BCCWJ, but the actual number will be much lower after removing false positives. There are 298 hits for "手伝う" and 1029 for "役に立つ".) I guess we don't really use "力になる" in the "practical help" sense in our everyday lives like the other two. Such uses of "力になる" tends to be reserved for less mundane situations, or more formal or affected style of speech, I may well say.

Answer 2

～の役に立つ just means "to be useful for X", whereas ～の力になる literally means "to empower X". The latter should be used only when something/someone makes someone feel stronger, encouraged or more capable. Also, the latter does not always mean something/someone is useful in a practical sense.

For example, この枕は快適に眠る力になる sounds odd because good sleeping has little to do with power or capability. 試合前にもらった彼の手紙は役に立った means his letter contained a practical advice and was useful, whereas 試合前にもらった彼の手紙は力になった means his letter encouraged you. On the other hand, 彼女は会社を成長させる役に立っている can sound rude and condescending, while 彼女は会社を成長させる力になっている is a nice expression.

手伝う is "to help" in the sense of "to lend a hand", and it never takes an inanimate thing as a subject.