Это класс, в котором мы пишем диктант.

В котором means in which.

It is made up of:

• в = in
• который = which / that

Here, который changes form to котором because:

1. it refers back to класс
2. класс is masculine singular
3. the preposition в expresses location here, so it requires the prepositional case

So:

• класс → masculine singular
• в котором → in which

A very literal structure is:

This is the classroom, in which we are writing a dictation.

Yes, very often you can.

A more conversational version would be:

Это класс, где мы пишем диктант.

Both are correct, but they feel a little different:

• где = more natural and common in everyday speech when talking about a place
• в котором = more formal, more explicit, slightly more bookish

So if you are just speaking naturally, где is often preferred. If you want to practice relative clauses, в котором is a very useful structure.

Because в котором мы пишем диктант is a relative clause describing класс.

Russian separates this kind of clause with a comma, just like English often does with structures like the room where... or the classroom in which...

So:

• Это класс = main part
• в котором мы пишем диктант = clause describing which classroom

That is why the comma is needed.

These two forms do different jobs.

• Это класс = This is a classroom / This is the classroom
• Этот класс = this classroom

In your sentence, это is being used in an identifying pattern:

• Это дом. = This is a house.
• Это школа. = This is a school.
• Это класс... = This is the classroom...

But этот класс is a noun phrase, not a complete identification by itself:

• Этот класс большой. = This classroom is big.

So Это класс, в котором мы пишем диктант means This is the classroom in which we write a dictation, not simply This classroom...

In Russian, the normal verb with диктант is писать — literally to write.

So:

• писать диктант = to write a dictation
• написать диктант = to write a dictation completely / finish writing it

This is just the standard Russian way to express the activity. Even if English might say do a dictation or talk about taking one in some contexts, Russian normally uses write.

Here пишем is from the imperfective verb писать, which fits well if the sentence describes an activity that happens there, or the process of writing it.

Диктант is the direct object of пишем, so it is in the accusative case.

For masculine inanimate nouns like диктант, the accusative singular looks the same as the nominative singular:

• nominative: диктант
• accusative: диктант

So the form does not change, even though the case function does.

Because Russian has no articles.

Russian does not have separate words for:

• a / an
• the

So Это класс could mean:

• This is a classroom
• This is the classroom

The exact meaning depends on context.

The same is true for диктант:

• we are writing a dictation
• we are writing the dictation

Context tells you which meaning is intended.

Here it means classroom.

Russian класс can mean different things, including:

• a classroom
• a school class / year group
• a class or category

But in this sentence, because of в котором and the idea of physically being in it, the meaning is clearly classroom.

So here:

• Это класс, в котором... = This is the classroom in which...

not This is the class in which... in the sense of a group of students.

The word order is fairly neutral.

• Это класс = This is the classroom
• в котором мы пишем диктант = in which we write a dictation

Inside the relative clause, мы пишем диктант is a normal, straightforward order:

• мы = subject
• пишем = verb
• диктант = object

Russian word order is flexible, so other orders are possible for emphasis, but this version is the most neutral and natural for learners to start with.

Yes.

Because пишем is imperfective present, it can mean either:

• we are writing a dictation right now, depending on context
• we write dictations there / that is where we do dictations, in a more general sense

In a sentence like this, many learners interpret it as a general description of the classroom: This is the classroom where we write dictations.

So the exact time reference comes from context, not from the verb form alone.