kono mondai ha zyugyou de sensei ni setumeisareru.

This problem will be explained by the teacher in class.

Breakdown of kono mondai ha zyugyou de sensei ni setumeisareru.

• は: topic particle
• 先せん生せい: teacher
• で: location particle
• この: this
• 問もん題だい: problem
• 説明せつめいする: to explain
• 授業じゅぎょう: class
• に: agent particle
• 〜される: passive form

Questions & Answers about kono mondai ha zyugyou de sensei ni setumeisareru.

Why is 説明される in the passive, and what is the active version of this sentence?

説明される is the passive form of 説明する (“to explain”).

• 説明する → “to explain”
• 説明される → “to be explained / will be explained”

The sentence is:
この問題は授業で先生に説明される。
Literally: “This problem, in class, by the teacher, will be explained.”

The corresponding active sentence would be:
先生は授業でこの問題を説明する。
“(The) teacher will explain this problem in class.”

Japanese often uses the passive when the focus is on the thing affected (ここでは この問題) rather than on the doer (先生).

Why is the teacher marked with に (先生に) instead of が like in the active sentence?

In a passive sentence in Japanese, the doer of the action (the agent) is usually marked with に.

• Active:

  • 先生がこの問題を説明する。
  • “The teacher explains this problem.”
  • The teacher (doer) → が

• Passive:

  • この問題は先生に説明される。
  • “This problem is explained by the teacher.”
  • The teacher (doer) → に

So 先生に corresponds to English “by the teacher” in a passive construction.

Why is この問題 followed by は and not by が or を?

In the active sentence, この問題 is the direct object, so it would take を:

• 先生は授業でこの問題を説明する。

When you turn this into a passive sentence, that direct object becomes the grammatical subject / main topic, so it is usually marked with が or, more commonly in context, は:

• この問題が授業で先生に説明される。 (grammatically fine)
• この問題は授業で先生に説明される。 (more natural if it’s the topic of discussion)

Using は makes この問題 the main topic of the sentence, something like “As for this problem…” or “This problem (in particular) will be explained…”.

What exactly does 授業で mean here, and why is で used instead of に?

授業で literally means “in class / during the class”.

The particle で is used to mark:

• The place where an action happens
• Or the situation / occasion in which something is done

So here 授業で = “in class” / “during the lesson”, i.e., the setting of the explanation.

に is used for other functions, such as:

• A specific point in time (３時に, “at 3 o’clock”)
• Destination / target (学校に行く, “go to school”)

For “doing something in class,” Japanese almost always uses 授業で, not 授業に.

Does 授業で mean exactly “in class” or more like “during class”?

It can be translated as either, depending on English style:

• “This problem will be explained in class.”
• “This problem will be explained during the lesson.”

授業で focuses on the context / occasion (the class session itself), so both “in class” and “during class” are valid natural translations. If you wanted to emphasize “during the period when class is happening,” you could also say 授業中に (“during class”), but in many cases 授業で is enough.

What tense is 説明される? Does it mean “is explained” or “will be explained”?

説明される is the non‑past plain passive form. Japanese non-past often covers both:

• Present/general: “is explained” / “is (usually) explained”
• Future: “will be explained / is going to be explained”

Which one is meant depends on context.
In a sentence like this (likely from a textbook, syllabus, or teacher’s explanation), it usually means:

“This problem will be explained in class.”

If you needed a clear past, you would use 説明された (“was explained / has been explained”).

Is the sentence polite or casual? How would it look in polite form?

説明される is the plain (dictionary) form, not the polite ます‑form. That means the sentence is neutral/plain in terms of politeness level:

• Plain:

  • この問題は授業で先生に説明される。

• Polite:

  • この問題は授業で先生に説明されます。

The passive itself doesn’t automatically make the sentence “polite,” but in practice, passive constructions are often used in more formal or written styles. Politeness is expressed mainly by choosing ～ます or plain form, plus word choice and context.

Can I change the word order, like putting 授業で or 先生に earlier in the sentence?

Yes, Japanese word order is fairly flexible as long as each noun + particle pair stays together and the verb comes at the end. All of these are grammatical (with slightly different emphasis):

• この問題は授業で先生に説明される。 (neutral)
• この問題は先生に授業で説明される。 (emphasis may lean slightly toward “by the teacher”)
• 授業でこの問題は先生に説明される。 (puts “in class” up front as the setting)
• 先生にこの問題は授業で説明される。 (starts with “by the teacher”)

The main thing is: don’t split 名詞 + 助詞 (noun + particle), and keep 説明される at the end.

Can some parts like 先生 or 授業で be omitted?

Yes. Japanese often drops information that is clear from context. For example:

• If it’s obvious the teacher will do it, you could omit 先生に:

  • この問題は授業で説明される。

• If it’s already understood the explanation happens in class, you could omit 授業で:

  • この問題は先生に説明される。

As long as the listener can infer the missing information from context, leaving things out is natural.

What is the difference in nuance between
この問題は授業で先生に説明される
and
先生は授業でこの問題を説明する?

Both can describe the same event, but the focus is different:

• 先生は授業でこの問題を説明する。

  • Focus is on the teacher’s action.
  • “The teacher will explain this problem in class.”

• この問題は授業で先生に説明される。

  • Focus is on the problem as something that will be dealt with.
  • “This problem will be explained in class (by the teacher).”
  • Sounds a bit more objective / like an announcement or description of the syllabus.

So if you’re talking about what the teacher will do, the active form is more natural.
If you’re talking about what will happen with this particular problem, the passive form fits better.

How do I conjugate 説明される (passive) into other common forms?

説明される is a regular ichidan‑type passive form of 説明する. Common conjugations:

• Non-past plain: 説明される – “is / will be explained”
• Past plain: 説明された – “was / has been explained”
• Non-past negative: 説明されない – “is not / will not be explained”
• Past negative: 説明されなかった – “was not explained”
• Polite non-past: 説明されます – “is / will be explained” (polite)
• Polite past: 説明されました – “was explained”
• て‑form: 説明されて – “being explained / and be explained / having been explained”

All of these are still passive; they just change tense, polarity, or politeness.

What is the nuance of 授業 compared to クラス in this kind of sentence?

Both can be translated as “class,” but the nuance differs:

• 授業

  • A lesson / class session itself (the activity: teaching + learning).
  • Very natural in school/university contexts, syllabi, explanations of what happens in lessons.
  • この問題は授業で説明される。 → sounds like something from a teacher or textbook.

• クラス

  • Often refers to:
    
    • A group of students (“my class”), or
    • A course (“I have a math class”).
  • クラスで can also mean “in class,” but 授業で is more “official lesson time” in many school contexts.

In a sentence describing what will be explained during a lesson, 授業で is the default, natural choice.