물이 고양이에 의해 엎질러졌어요 – the water was split by the cat.
Here, 에 의해 denotes the cause of what happened. However, it seems that we can just use '에' for a similar meaning, e.g. 도둑이 경찰에게 잡혔어요 – the thief was caught by the police.
In the first sentence, would there be any difference in register or meaning if we just said:
물이 고양이에 엎질러졌어요?
If not, would omission of '의해' make a difference in any other sentences?
"물이 고양이에 엎질러졌다" would mean "Water was spilled on the cat".
엎질러지다 is mostly used like this:
물이 바닥에 엎질러지다.
모래 위에 엎질러진 좁쌀
where the adverb "~에" indicating the surface that the thing was spilled onto.
You can use the adverb as the performer, but only like this (when it's impossible to interpret the adverb as the surface):
밥상이 명훈의 발길질에 엎질러졌다.
밥상이 명훈에 엎질러졌다. (X) -> this would mean "the table was flipped onto 명훈."
Or you can explicitly state that 명훈 is the performer, by using 의해.
밥상이 명훈에 의해 엎질러졌다.
But Korean doesn't use these kinds of sentences a lot, so it sounds like a badly translated English sentence. It's best to not use the passive in these sentences.
So alternatively, for the cat sentence, you can write it like this:
물이 고양이의 뜀박질에 엎질러졌다.
물이 고양이에 의해 엎질러졌다 (Not recommended)
or just don't use the passive, which sounds the most natural:
고양이가 물을 엎질렀다.
I recently did a lot of research into this area, talked to a lot of people (natives as well as true experts including one professor with a Ph.D), read a lot of material, and looked at several dozen sentences. Here is the outline of the situation...(Please note that there will always be exceptions to these generalities and I have seen them, but only a few so far.)
Firstly, most of the time 에 의하여/해 will work, but it is not always the natural choice and occasionally it seems to be wrong.
When deciding whether to replace that with 에게, the following guidelines fit in most cases:
에게 can be used if both parties are animate, or animate-like (such as countries, companies, etc.)
에게 can be used if the verb is "sensory" (relating to hearing, seeing, discovering, feeling, etc.)
한테 can replace 에게 in some dialectical and/or spoken uses. I was told that people in Gyeongsang (SE part of Korea) are more likely to use this, for example, when speaking. Not sure otherwise, but 에게 is a safer bet.
One thing that I'm mystified about is the difference between 에 and 에게 in passive sentences. Grammar books say that 에게 should be used for animate agents, and 에 for inanimates. However, there are many examples "out there" of 에 used with living things, and also 에게 used with non-living things, so it is truly confusing to me.
Since the above generalities pretty much only make sense with animates, there is more to the story. But 에 의해 seems to be the safest and most general choice for inanimates.
Of course, 에 is used for "by" in more general cases of "cause or means" (e.g. 병에 죽었다) so I think there some overlap here...I plan to look much closer at this in the near future.
I'm still waiting for some feedback from a couple of my best contacts on this, so I might edit this response later a little bit.
