Vi spiser frokost i kantinen, og derefter går jeg tilbage til kontoret.

Because the two clauses have different subjects:

• Vi spiser frokost i kantinen = We eat lunch in the canteen
• og derefter går jeg tilbage til kontoret = and afterwards I go back to the office

So the speaker is part of the group having lunch, but only I am mentioned as going back to the office.

In Danish, meals are often used without an article, just like in English expressions such as have lunch.

So:

• spise frokost = eat lunch / have lunch
• spise en frokost would sound more like eat a lunch, which is less natural unless you are talking about one specific lunch as an object.

This is very common with meal words:

• spise morgenmad = eat breakfast
• spise frokost = eat lunch
• spise aftensmad = eat dinner

I kantinen literally means in the canteen.

• i = in
• kantinen = the canteen

Here i is used because the idea is being inside the canteen. Danish often uses i for being in an enclosed place.

Also notice kantinen ends in -en, which makes it definite:

• en kantine = a canteen
• kantinen = the canteen

The comma is there because the sentence joins two clauses:

• Vi spiser frokost i kantinen
• og derefter går jeg tilbage til kontoret

In Danish, commas are often used before og when it connects two full clauses with their own verbs and subjects. Comma usage can vary a bit depending on style and comma system, but this punctuation is completely normal.

Derefter means after that, afterwards, or thereafter.

In this sentence, it links one action to the next:

• first: eating lunch
• then: going back to the office

Compared with similar words:

• derefter = afterwards / after that, a bit more formal or written
• bagefter = afterwards / later, very common in speech
• så = then / so, often simpler and more conversational

So all of these can sometimes work, but derefter sounds a little more structured.

This is because Danish follows the V2 rule in main clauses. That means the finite verb usually comes in the second position.

In the second clause, derefter is placed first:

• og derefter går jeg tilbage til kontoret

So the order becomes:

1. derefter
2. går
3. jeg

If the subject comes first, then the order is:

• Jeg går tilbage til kontoret derefter

But when another element is moved to the front, the verb must still stay in second position, so the subject moves after the verb.

Tilbage means back.

So:

• går til kontoret = go to the office
• går tilbage til kontoret = go back to the office

It shows that the speaker is returning to a place they were at before. In English, back works the same way.

Because til expresses movement toward a destination:

• til kontoret = to the office

Compare:

• Jeg går til kontoret = I go to the office
• Jeg er på kontoret = I am at the office
• Jeg sidder i kontoret = I am sitting in the office

So the important distinction is:

• til = direction / movement
• i / på = location

Because kontoret is the definite singular form of kontor.

• et kontor = an office
• kontoret = the office

Danish usually adds the definite article as an ending rather than using a separate word like English the.

For neuter nouns like kontor, the definite ending is often -et:

• et hus → huset
• et bord → bordet
• et kontor → kontoret

They are both in the present tense:

• spiser = eat / am eating
• går = go / am going

In Danish, the present tense can cover several meanings that English often separates:

• habitual: We eat lunch in the canteen
• current or narrative present: We are eating lunch in the canteen
• sometimes near future, depending on context

Also, Danish verbs do not change for person:

• jeg spiser
• du spiser
• vi spiser

The verb form stays the same.

Usually, yes. Frokost normally means lunch, especially the midday meal.

In Danish culture, frokost can also sometimes suggest a meal with open sandwiches or a more traditional lunch setting, depending on context, but for learners the safest meaning is simply lunch.

Yes, that is grammatically possible.

Compare:

• og derefter går jeg tilbage til kontoret
• og jeg går derefter tilbage til kontoret

Both mean roughly the same thing. The difference is mostly about emphasis and flow:

• derefter first: emphasizes the sequence, afterwards
• jeg first: sounds more neutral, with the subject in normal first position

The original version is very natural because it highlights the order of events.

It is natural and correct. It sounds clear and slightly tidy or written because of derefter.

In everyday speech, many people might say something a little more casual, such as:

• Vi spiser frokost i kantinen, og bagefter går jeg tilbage til kontoret.
• Vi spiser frokost i kantinen, og så går jeg tilbage til kontoret.

But the original sentence is absolutely good Danish.