grammar - Passive form - The exact difference between を and が

I've seen this question asked before, but i feel the answer didn't quite answer all my questions, so here goes.

Now, I am rather sure that this:

ケーキが食べられた

Means:

The cake was eaten. (by someone)

Now, recently I've noticed a few cases where the passive form was used with "を" instead of "が".

Judging from the examples I've seen, I would guess that:

ケーキを食べられた

Means something along the lines of

my cake was eaten (by someone)

However, can this be used with someone elses cake instead of my own? I guess what I'm basically asking is whether this is possible:

犬は猫にケーキを食べられた

And if it is, does it mean:

The dog's cake was eaten by the cat

Or can the Aを(passive verb) construction only be used when I'm speaking of something directly relating to myself, the speaker?

Answer

Yes, your reasoning is correct. は／が is used to describe when the action happens to the thing itself. を is used to emphasize the (usually negative) effect of the action on the subject, optionally indicating the agent of the action with に.

• 弟にケーキを食べられた　→　My cake was eaten by my little brother (anger/aggravation implied).


• カバンを取られてしまった　→　My bag was taken/stolen

You could replace the を version with の〜は／が version, but 1) I'm not sure if this is grammatically correct, 2) it's not as strong of a statement, and 3) may not convey the negativity as the former does.

• 私のケーキは食べられた　→　weaker, possibly a neutral statement; may not be acceptable


• 私はケーキを食べられた　→　stronger; definite negativity included

The English translation might include the word "on", although it might not make sense all the time. Though personally, it helps me to remember it this way.

• 電車の中の人に足を踏まれました　→　My foot was stepped on by a person in the train ("on" works in this translation).


• 私は弟にケーキを食べられた　→　My cake was eaten "on" by my little brother ("on" doesn't work in this way in English, though it still kinda makes sense).

Note that with the は／が pattern to describe an action on the subject itself, there are nuances about when to use the passive form of the action and when to use the intransitive verb of the action (if it exists) as I discuss in this question.

Also, you probably shouldn't be feeding cake to your dog. ;)