Kad pišeš službenu poruku, piši što pristojnije i što jasnije.

They have different grammatical jobs:

• pišeš = you write / you are writing
  This is 2nd person singular present tense.
• piši = write!
  This is the imperative.

So the sentence is structured like this:

• Kad pišeš službenu poruku = When you write a formal message
• piši što pristojnije i što jasnije = write as politely and as clearly as possible

A very common Croatian pattern is:

• Kad + present tense, imperative

for general instructions like When you do X, do Y.

Yes, here kad means when.

In this sentence, it introduces a general situation:

• Kad pišeš službenu poruku...
  = When(ever) you write a formal message...

So it is not only about one specific moment. It has a general meaning, like giving advice.

You can often also use kada, which is a slightly fuller form:

• Kad pišeš...
• Kada pišeš...

Both mean when. In everyday speech, kad is very common.

Because it is the direct object of pišeš / piši.

The basic noun is:

• poruka = message

Here it must go into the accusative singular because it is the thing being written:

• nominative: službena poruka
• accusative: službenu poruku

The adjective službena also changes to match the noun:

• službena poruka = a formal/official message
• pišeš službenu poruku = you are writing a formal/official message

So both words change because they are feminine singular accusative.

It can suggest both, depending on context.

• služben often relates to something official, businesslike, or formal
• službena poruka is best understood here as a formal message or official message

In this sentence, the idea is clearly not a casual text to a friend, but communication in a professional, administrative, or otherwise formal context.

This is a very important Croatian pattern:

• što + comparative

It means:

• as ... as possible
• as ... as you can

So:

• što pristojnije = as politely as possible
• što jasnije = as clearly as possible

Here što is not the pronoun what. It is part of a fixed intensifying structure.

So the sentence means:

• Write as politely and as clearly as possible.

Here they function as adverbs, because they describe how you should write.

Compare:

• pristojan = polite
• jasan = clear

Their comparative forms are:

• pristojnije = more politely / more polite
• jasnije = more clearly / clearer

In this sentence they modify the verb piši:

• Piši kako?
  pristojnije, jasnije

So in English we naturally translate them as adverbs:

• politely
• clearly

Because Croatian uses the comparative after što in this pattern:

• što + comparative = as ... as possible

So you do not say:

• što pristojno
• što jasno

You say:

• što pristojnije
• što jasnije

This is similar to how English says:

• as politely as possible
• as clearly as possible

not just

• politely
• clearly

Not in the same way, and also note the correct form would be najjašnje only if it existed regularly, but with jasno the normal superlative is najjašnije? Actually, in standard usage the comparative/superlative system here is not what this sentence needs.

The key point is:

• što pristojnije / što jasnije = as politely / clearly as possible
• naj- forms mean the most ...

Those are not the same idea.

Croatian normally expresses as ... as possible with:

• što + comparative

So this sentence is correct and idiomatic as it stands.

Repeating što is normal and very natural:

• što pristojnije i što jasnije

It clearly applies the same as ... as possible meaning to both words.

You may sometimes hear reduced phrasing in speech, but for learners it is best to keep both:

• što pristojnije i što jasnije

That is the clearest and safest version.

Yes.

Both verb forms are 2nd person singular informal:

• pišeš = you write
• piši = write!

So the speaker is talking to one person using ti.

If you wanted the polite/formal or plural form, you would use:

• Kad pišete službenu poruku, pišite što pristojnije i što jasnije.

That can mean:

• speaking politely to one person (Vi)
• or addressing several people

Because the first part is a subordinate clause introduced by kad:

• Kad pišeš službenu poruku, ...

Then comes the main clause:

• ..., piši što pristojnije i što jasnije.

Croatian normally separates this kind of introductory clause with a comma, just like English often does:

• When you write a formal message, write as politely and as clearly as possible.

This is about aspect.

• pisati = imperfective = to write, to be writing, to write in general
• napisati = perfective = to write / finish writing

The imperative piši from pisati is natural here because the sentence gives a general manner instruction:

• When writing a formal message, write politely and clearly.

It is about how to write, not about completing one single message.

If you used napiši, the focus would shift more toward completing a specific act of writing, which is not the main idea here.

Yes, that is a very good way to understand it.

In this sentence:

• Kad pišeš službenu poruku...

the meaning is very general, so English could also express it as:

• Whenever you write a formal message...

That fits the instructional tone very well.

Poruka is a broad word meaning message.

Depending on context, it could refer to:

• a written message
• an email
• a text/message in an app
• some other short written communication

Because of službenu, the phrase suggests a formal written communication, but it does not specify exactly which medium. If Croatian wanted to be more specific, it might use words like:

• e-poruka / email
• pismo = letter

So službenu poruku is a fairly general expression.

Word-for-word, parts of it may look like that, but that would miss the idiomatic meaning.

Because of the pattern što + comparative, the natural meaning is:

• as politely as possible
• as clearly as possible

So the best translation is:

• When you write a formal message, write as politely and as clearly as possible.

or

• Whenever you write a formal message, write as politely and clearly as possible.

To some extent, yes, but this version is very natural and neutral:

• Kad pišeš službenu poruku, piši što pristojnije i što jasnije.

Croatian word order is more flexible than English, but not every rearrangement sounds equally natural. This sentence has a clear instructional flow:

1. situation: Kad pišeš službenu poruku
2. advice: piši što pristojnije i što jasnije

That is why it works well as a model sentence for learners.