Jeg er i gang med at rette mit udkast, så jeg svarer først senere i aften.

Er i gang med is a very common Danish expression meaning to be in the middle of doing something or to be currently working on something.

So:

Jeg er i gang med at rette mit udkast
= I’m in the middle of revising/editing my draft

It is often used where English would use a progressive form like I am working on... or I am busy...

A few similar examples:

• Jeg er i gang med at lave mad = I’m cooking / I’m in the middle of making food
• Hun er i gang med lektierne = She is working on her homework

Because after i gang med, Danish usually uses:

• i gang med + noun
• i gang med at + infinitive

Here, rette is a verb, so it needs at:

• i gang med at rette = in the middle of correcting/editing

Compare:

• Jeg er i gang med arbejdet = I’m working on the task/work
• Jeg er i gang med at arbejde = I’m working

So at is there because rette is being used as an infinitive verb.

Here rette means something like:

• correct
• revise
• edit

The exact English translation depends on context.

In this sentence, rette mit udkast most likely means:

• revise my draft
• edit my draft
• possibly correct my draft

So it does not just mean fixing mistakes in a narrow grammar sense. It can also mean improving or adjusting written work more generally.

Because udkast is a neuter noun in Danish.

Danish possessives must match the grammatical gender of the noun:

• min for common gender nouns
• mit for neuter nouns
• mine for plural nouns

So:

• en bog → min bog
• et udkast → mit udkast
• mine bøger

Since the noun is et udkast, the correct form is mit udkast.

Udkast means draft.

It can refer to a first or unfinished version of something, especially something written, such as:

• an email
• an essay
• a report
• a message
• an article

So mit udkast means my draft or the draft I’m working on.

Because Danish often uses the present tense to talk about the future when the time is clear from context.

In this sentence:

• jeg svarer literally looks like I answer / I am answering
• but senere i aften makes the future meaning clear

So:

jeg svarer først senere i aften
means
I’ll reply later tonight

This is very normal in Danish.

Other examples:

• Vi ses i morgen = We’ll see each other tomorrow
• Jeg kommer klokken otte = I’m coming at eight

Here så means so or therefore.

It links the two ideas:

• I’m revising my draft
• so I’ll reply later tonight

So the sentence structure is:

Jeg er i gang med at rette mit udkast, så jeg svarer først senere i aften.

That means:

I’m in the middle of revising my draft, so I won’t reply until later tonight.

This så is not the same as the så meaning then in every context, although in English the ideas can overlap.

Because så here is connecting two main clauses, and the second clause keeps normal main-clause word order.

The second clause is:

• jeg = subject
• svarer = verb

So:

så jeg svarer først senere i aften

That is normal Danish word order.

You may also see Danish main clauses with another element placed first, which causes inversion:

• Så svarer jeg senere i aften = Then/So I’ll reply later tonight

But in your sentence, the subject comes first in the second clause, so the order is simply jeg svarer.

Først adds the meaning of not until.

So:

• jeg svarer senere i aften = I’ll reply later tonight
• jeg svarer først senere i aften = I won’t reply until later tonight

It emphasizes that the reply will not come earlier.

This is very common in Danish. For example:

• Jeg kommer først i morgen = I’m not coming until tomorrow
• Vi er færdige først klokken ni = We won’t be finished until nine

So først is important here because it strengthens the idea of delay.

Senere i aften means later this evening / later tonight.

Breakdown:

• senere = later
• i aften = tonight / this evening

In English, whether you translate i aften as this evening or tonight depends on tone and context. In this sentence, later tonight sounds very natural.

Yes, it is normal here because the sentence has two coordinated clauses:

• Jeg er i gang med at rette mit udkast
• så jeg svarer først senere i aften

The comma helps separate the two parts clearly.

For a learner, the most useful thing is simply to notice that Danish often uses commas to divide clauses in longer sentences like this one. So the comma here is standard and natural.