I was stolen my chocolate suitcase! [Japanese]

を

This is the particle that, along with に, separates the passives from the people that get passive done to them. What? Sorry, that's the way it works in Japanese. To explain the misnomered "suffering passive," I'm going to make a few sentences that have probably never been made in Japanese before, -- inspired by both Ray Romano and Jay Rubin (see refs at the bottom of the post)-- yet I am pretty confident I can demonstrate their meaning to you and show you why を is important. Consider:

チョコ鞄を盗んだ [sub.] stole the chocolate suitcase
チョコ鞄が盗まれた the chocolate suitcase was stolen
チョコ鞄を盗まれた [sub. underwent the experience of] the chocolate suitcase was stolen [by agent ～]
Continue this post...
If we take out the bracketed words, we see that 2 and 3 look very similar. The difference is を (the difference between 1 and 3 is the passive conjugation). を always marks an object. If there is an object being discussed, that means we are not talking about the object (chocolate suitcase), because if we were talking about the object, we would be using が (or maybe は), which is traditionally called the subject marker (and thus the object would not be called an object any more). The "subject marker" terminology is up to debate, but が's function is to say, "hey, lets talk about this word I am attached to!" Where as を is like, "somebody is using this word, but it's not the subject nor the agent. Also, I like oranges." And then everybody is like, "Japanese is strange, what is this orange business? There is nothing about oranges anywhere I can see!" And I'm like, "I just threw that in to be funny." And then everybody is like, "stop being cute, you are only making a confusing thing more confusing." And I'm like, "Well, excuuuuuse me, princess."　Anyways, if を shows up around the passive conjugation, you've got the suffering passive on your hands.

Let's talk about に too; it's pretty important, and in sentences where we don't have an object, it can be our only clue. に in these sentences is going to be like the English "by". 建物を彼に建たれた。(the building was built by him [and hey, it's blocking my view! etc...]). によって is another form of the same particle, used to indicate that such and such was done by so-and-so, and can show up in either the suffering or not suffering passives. So if you remember を="not subject but object" and that に="by", it helps to keep things straight. Sometimes even Japanese people screw this up and put a が in the place of に or something. That's life. My grammar be unperfect toos. から is another alternate に, but I am honestly not sure it meshes with the suffering passive; I saw no examples. I had a mnemonic "鬼 ("をに") make me suffer" for this grammar, but it didn't help until I really pounded down what kind of words the particles attach to, as well as what order those words are supposed to be in (not 〜を〜に, but I'll cover that in the rules section).

So as usual, it's the English terminology that's to blame for most of our confusion on this construction. Usually, this "agentにobjectをverbられる" thing is called the "suffering passive" and why not? After all, it uses the passive conjugation and usually we suffer from people messin' with our stuff. Well, suffer may not always mean something bad is happening to you; there seems to be some archaic neutralness that we English speakers don't usually think of any more. So that's why not. Basically think suffer="experience/undergo/feel the influence of", and it may clear things up for you. We could all try calling it the experiential-passive, but seeing as this is a small blog, I don't expect the term to catch on any time soon. I've seen one or two people use terms like "affected passive". The terminology doesn't seem to have too much convention about it in Japanese. I saw 間接受け身 (see references below) and some others, but it's a safe bet that Japanese people will have no idea what you are talking about no matter what grammatical term you bust out. In any case, it is really easy to screw up, but I do recommend learning how to recognize it.

The Rules

According to one of my books, there are are two basic types of passive suffering sentences:
Type 1: Agent influences the subject "unexpectedly" and uses intransitive verbs to do it.を is missing, because it doesn't like working with intransitive verbs I guess.
Pattern 1: "subjectは agentに verb-passive"
例文：私は妹に泣かれて、嫌だった。
例文：旅行中、ずっと雪に降られました。
Rule: The sentence pattern cannot change order from the set pattern "subjectは agentに verb-passive", though I'm sure we can leave things out as in normal Japanese if everyone knows what is being talked about. This set order thing is different from normal, fluid Japanese, and I suspect that it's because it's both a grammar and an idiom.

Type 2: Agent influences the subject's body parts or belongings.
Pattern 2: "subjectは agentに objectを verb-passive" (give or take understood parties)
例文：(私は、)電車の中で、誰かに足を踏まれました。
Rule: Don't の the subject's body parts or belongings! It sounds odd. The following is wrong:
誤用例：　バスの中で、私の腕は、女性に噛まれた。

Other rules:
An object is not normally used as the subject in passive sentences, suffering or otherwise.
誤用例：書類は山下から山口に渡されました。

If you become an agent, the sentence shouldn't be passive.
誤用例：ボッブは私に蹴られた。

Examples, and grains of salt to take them with

As is par for the course, subjects and agents get dropped all the time in Japanese. I didn't use a single one in those example sentences about suitcase of delicious, delicious chocolate, and yet we can still make pretty good guesses about who is involved. It's all about context, baby. Plus, the subject is almost always the speaker; this is mostly a 表現 to complain after all. Here are some sentences with subjects and/or agents sprinkled in. Drag your mouse over the blank spaces to see my hidden translations/notes (you may be surprised by what you thought you just read! Keep in mind that what is passive in Japanese may not be so in English). Comment if you disagree; I only nailed this form down recently, and yes, that means I am long overdue; it is the ones where "I" am not the subject that I feel the shakiest about.

私は一晩中子供に泣かれて、困った。I had to endure the child crying throughout the night; what a pain! [pattern/type 1 here. theは, に and passive form are the important clues, as is the 困った. Such 迷惑 type words often accompany the form. Maybe the unsaid object could be tears or wails, or maybe it just doesn't exist ever in pattern 1, I'm not sure.]

私はボッブサップに足を折られてしまった。 I had my leg broken by Bob Sapp. [Notice we further conjugated the end of our passive conjugation to the gerund (て) conjugation to attach しまう. I read that しまう is used for its own passive conjugation, but only in the English sense of the word so far as I know, and it doesn't matter here.]

私がボッブサップに足を折られてしまった。 I had my leg broken by Bob Sapp. [Just more emphasis on me, me, me.]

誰かに犬を食べられた。My dog was eaten by someone. [Well that is a rare situation, but I can see how it would affect the master. The dog is the object, so it is the thing eaten, poor thing. If we want someone to suffer us feeding the dog, we are gonna need some food to give it...]

犬は誰かにチョコを食べさせられた。The dog was fed chocolate by someone. [The dog is the subject. This passive conjugation of a causative conjugation is pretty rare and seems to make Nipponese people tilt their heads in confusion; use with care. I waffled a lot about whether the dog could be the subject; I'm pretty sure it is, because that's what the particles are telling me. There is one other thing about this sentence that will make it risky: We have a hard time knowing that a dog was troubled (unless we are dog whisperers), but I think the chocolate makes this a safe sentence, since the dog will die.]

僕は子供に鼻をほじられて、キモかった。　The kid picked [my?] nose! That was grooss! [I really wanted to say, "The child picked it's nose before me; it was gross. " but the rules prevent me from saying that edit:a nihonjin got the idea anyways in the comments. Anyways, 僕は子供が自分の鼻をほじることに見られた。seems to get at what I want, it's just a little more complicated with "the kid's nose picking" acting as an agent. Very spurious.]

学校のパソコンを壊されたのだ？僕は首だ！ The school computer has been broken? I'm gonna be fired! [This one may break the rules! :/ We may be able to get away with the school のing it (is it possessive, or part of the object's name?), but I am going to have to check with some Nihonjin on that point. I think it may be alright, because "I" am the one who is gonna get fired over it. Or it may just mean that this is not a suffering passive, but otherwise valid. edit: confirmed okay.]
あたしはClaytonian様にキスをチューとされた。I was kissed by Mr. Claytonian. [see, it's not always about suffering, she is probably just hinting that she fell deeply in love, alas.]

In conclusion, you may want to limit your use of this form since the rules are so restrictive.
So next time I may try to wrap my head around the causative-passive construction some more. What does it mean for mankind's future? And how does polite-passiveness work?* Will Diana get married? Who was James shot by, which I suffered from? Stay tuned.
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References:
Ray Romano claimed the sentence "let go of my chocolate suitcase" was a sentence that, while possible, had never been uttered.

Jay Rubin wrote Making Sense of Japanese, which I should take and use for a holiday around a set date where I review it once a year. Same goes with Tae Kim's guide.

Speaking of which, Tae wrote this article on the subject. He is never wrong.

I also found this, this, this, this and this.

From this page we get a good description, in Japanese, of the grammar:

間接受身は「子供が私のパソコンをこわす」から「私は子供にパソコンをこわされる」が表現できるように、主語（私）が誰か（子供）によって、直接的な動作・行為を受けたのではないが、パソコンをこわされることによって、間接的に何らかの影響（多くの場合被害）を受けることを表す受身です。

Thanks to the Lang-8 people that made me realize that I tend to mix my sounds up and conjugate things into the causative at times! And I wouldn't have realized I needed to finally figure this grammar out if they hadn't been diligently beating me down every time I used the particles incorrectly.

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*It gives me a headache, that's how it works.

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Nobody is allowed to quibble with me on whether the things followed by は and が in any of this were "subjects" or not; that distinction is too expansive to consider in this post.