Jag sitter i klassrummet och skriver i mitt block.

Because Swedish often uses a posture verb to describe what someone is doing.

• sitter = is sitting / sits
• So Jag sitter i klassrummet och skriver ... literally means I am sitting in the classroom and writing ...

In natural English, we might just say I’m in the classroom writing in my notebook, but Swedish often likes to mention the physical position:

• sitter = sitting
• står = standing
• ligger = lying

So this sentence does not just say that the person is writing; it also tells you they are doing it while seated.

Because Swedish normally uses och + a second finite verb in this kind of expression.

So:

• Jag sitter och skriver = I’m sitting and writing

Both verbs are in the present tense:

• sitter
• skriver

This is a very common Swedish pattern. It often works like an English progressive expression, even though Swedish does not have a special -ing tense the way English does.

Not usually. Swedish normally uses the present tense for both:

• I write
• I am writing

So skriver can mean either one, depending on context.

In this sentence, sitter ... och skriver strongly suggests something happening right now, so English naturally translates it with am writing.

Because klassrummet is the definite form: the classroom.

The noun is:

• ett klassrum = a classroom

Since klassrum is an ett-word, its singular definite form is:

• klassrummet = the classroom

So:

• i klassrummet = in the classroom

i usually means in when you are inside something enclosed, such as a room.

So:

• i klassrummet = in the classroom

That is the normal preposition here because a classroom is a room you are physically inside.

Because in Swedish, you normally write in a notebook, pad, or exercise book.

So:

• skriva i ett block
• skriva i en bok
• skriva i ett häfte

This is very natural in Swedish.

English often says write in a notebook too, so this part matches English quite well.

By contrast, Swedish would often use på with things seen more as surfaces:

• skriva på ett papper = write on a piece of paper
• skriva på tavlan = write on the board

Because block is an ett-word.

In Swedish, possessive words change depending on the gender/number of the noun:

• min for en-words
• mitt for ett-words
• mina for plural nouns

Since it is:

• ett block

you must say:

• mitt block = my notebook / my pad

Not min block.

Because after a possessive, Swedish normally uses the noun in the indefinite form.

So you say:

• mitt block
• min bok
• mina pennor

not:

• mitt blocket
• min boken
• mina pennorna

The possessive already makes the noun specific, so Swedish does not also add the definite ending.

Here block means something like:

• notebook
• notepad
• writing pad

It does not mean an English block in the sense of a chunk of wood or a city block.

A more explicit word is anteckningsblock, but block is very common in everyday Swedish.

Because the subject is the same for both verbs, and Swedish usually does not repeat it unnecessarily.

So:

• Jag sitter i klassrummet och skriver i mitt block.

is perfectly natural.

You could say:

• Jag sitter i klassrummet och jag skriver i mitt block.

But that sounds heavier and less natural unless you want extra emphasis.

Yes. That would simply mean:

• I’m writing in the classroom
• or I write in the classroom, depending on context

But Jag sitter i klassrummet och skriver ... gives more detail. It tells us:

1. the person is in the classroom
2. they are sitting
3. they are writing

So the original sentence is a bit more vivid and specific.