Vor dem Doppelpunkt lasse ich kein Leerzeichen, aber danach schon.

Because vor is a two-way preposition in German: it can take either the dative or the accusative.

Here, vor means before/in front of in a fixed, non-moving sense, so it takes the dative:

• vor dem Doppelpunkt

Also, Doppelpunkt is masculine (der Doppelpunkt), so its dative singular form is dem.

Doppelpunkt is the normal German word for a colon (:).

Literally, it means double point:

• doppel = double
• Punkt = point/dot

So der Doppelpunkt is the punctuation mark :.

Yes. Here lassen means to leave.

So:

• Ich lasse kein Leerzeichen. = I leave no space.

This is a natural way in German to talk about spacing in writing or typography. English often says I don't put a space, but German commonly says I leave no space.

Because Leerzeichen is a noun, and German usually uses kein to negate nouns when English would use no.

So:

• kein Leerzeichen = no space

Compare:

• Ich habe kein Auto. = I have no car.
• Ich lasse kein Leerzeichen. = I leave no space.

Nicht is used more for negating verbs, adjectives, adverbs, or whole statements.

It is accusative singular neuter.

Why?

• The verb lassen takes a direct object
• Leerzeichen is neuter: das Leerzeichen
• In the accusative singular neuter, kein stays kein

So:

• nominative: kein Leerzeichen
• accusative: kein Leerzeichen

That is why the form does not change here.

Here danach means after that or after it, referring to the colon in the text.

In this sentence, it is about position in writing, not really time in the usual sense. So:

• vor dem Doppelpunkt = before the colon
• danach = after it / after the colon

German often uses danach this way when referring back to something just mentioned.

Right — here schon does not mean already.

In this sentence, schon adds a contrast and means something like:

• indeed
• actually yes
• on the other hand, yes

So:

• aber danach schon = but after it, I do

It contrasts with the earlier negative statement:

• kein Leerzeichen before it
• schon after it

This use of schon is very common in spoken and informal written German.

Because German often omits repeated words when they are easy to understand from context.

The full version would be something like:

• Vor dem Doppelpunkt lasse ich kein Leerzeichen, aber danach lasse ich schon ein Leerzeichen.

But that sounds repetitive, so German shortens it to:

• ..., aber danach schon.

This kind of ellipsis is very natural.

German allows different elements to come first in the sentence. Whatever comes first is followed by the finite verb in second position.

So:

• Vor dem Doppelpunkt = first element
• lasse = verb in second position
• ich = subject after the verb

That gives:

• Vor dem Doppelpunkt lasse ich kein Leerzeichen

A more neutral version would be:

• Ich lasse vor dem Doppelpunkt kein Leerzeichen

Both are correct, but starting with Vor dem Doppelpunkt puts more focus on the punctuation rule itself.

Yes. Das Leerzeichen is singular, and here it means a space in the sense of a single blank space character.

So:

• kein Leerzeichen = no space
• ein Leerzeichen = a space

That is why the singular is used rather than the plural Leerzeichen.