Glad I could be of some help
The honorific use of passive is important too. One thing to keep in mind is that passive forms are less polite than using honorific verb forms (the お〜になる construction) and the special honorific words (things like いらっしゃる for いる) The reason the passive form conveys politeness is because the passive construction is more indirect in expressing things than direct sentences, and indirectness is the major method of expressing politeness in Japanese.
One thing that can be confusing for learners is the fact that the passive form for verbs like 食べる is identical to the potential form. This means those verbs can potentially be interpreted three ways: as potential verbs, honorific, or passive. You can determine which based upon the context of the sentence, as well as the particles. Also, that problem can be avoided if one uses the abbreviated potential (食べれる instead of 食べられる as the abbreviation of れる instead of られる can only be done for the potential form, not the passive)
For situations where it's identical, however, here's how you can figure it out:
先生は納豆
が食べられる
- If が is marking the direct object, then it has to be potential form.
先生は納豆
を食べられた
- If を is marking the direct object, and there's no agent marked by に、then the sentence can be either passive, honorific, or potential. Only context will let you know for sure (though if the verb is in the past tense it's likely either honorific or passive, as people don't too often talk about how they USED TO be able to do an action unless the context is right. Hearing "I can eat sushi" is far more common than "I used to be able to eat sushi" for example.) And if the subject of the sentence isn't someone that the speaker is likely to be talking about in honorifics, then it's pretty much guaranteed to be passive (If the dialogue is a friend talking about a friend they're not going to be using honorifics to describe their friend's actions)
先生はゴジラ
に納豆を食べられた
- Here we have に marking an agent, which means that the sentence has to be passive.
Additionally, something that teachers and textbooks don't usually cover about the passive form, because they don't want to confuse students, is that other things than に can mark the agent in passive form, so don't be too surprised if you see it happening.
If the agent is a source-in other words there is something that is being transmitted or coming from the agent-then から can mark the agent instead of に since から can indicate sources.
To use DOBJG's examples:
私は学生から日本の大学の事を聞かれた
- The action of "asking" is coming FROM the students, so the students are the source of the action, and thus can be marked with から.
木田さんは皆から尊敬されている
- The action of "respecting" is coming FROM everyone, so they are the source.
This isn't mentioned in the DOBJG, for whatever reason, but で can also be used to mark agents. Primarily this is used for when the agent isn't a person, but is an entity like a corporation or institution. If you say, "I was asked by the software company to write a new program" then you could mark the software company with で for the passive form.
Use of から to mark agents can be tricky, because what verbs allow for "sources" isn't identical between Japanese and English. I would suggest only mimicking the usages of it you encounter yourself, and until you feel very comfortable with using で or から just stick with に as it's the safest and easiest choice.
I thought Japanese grammar is "easy"~
