U poruci često napišem kraticu "npr.", ali u službenom e-mailu radije pišem cijeli izraz.

Because the preposition u means in here, and with that meaning it takes the locative case.

• poruka → u poruci
• službeni e-mail → u službenom e-mailu

So both phrases mean in a message and in a formal email.

A useful contrast:

• u + locative = location or context: in
• u + accusative = motion into: into

So:

• u poruci = in the message
• u poruku = into the message

Because kraticu is the direct object of napišem, so it is in the accusative singular.

The noun is:

• nominative: kratica
• accusative: kraticu

This is very common with feminine nouns ending in -a:

• knjiga → knjigu
• poruka → poruku
• kratica → kraticu

Yes, it is also accusative, but for many masculine inanimate nouns, the accusative looks exactly the same as the nominative.

So:

• nominative: izraz
• accusative: izraz

That is why you see:

• pišem cijeli izraz

The case changed grammatically, but the form did not change visibly.

npr. stands for na primjer, which is the Croatian equivalent of for example or e.g.

In writing, npr. is very common. In speech, people usually say na primjer rather than reading the letters one by one.

So in this sentence, the contrast is:

• informal message: use the abbreviation npr.
• formal email: write the full phrase na primjer

This is about verbal aspect, which is very important in Croatian.

• pisati / pišem = imperfective
  Focuses on the activity or habit of writing.
• napisati / napišem = perfective
  Focuses on writing something as a completed act.

So the sentence creates a subtle contrast:

• često napišem kraticu = I often end up writing the abbreviation / I often write it down as a complete item
• radije pišem cijeli izraz = I prefer writing the full expression / my general habit or preference is to write it out

For an English speaker, both may just look like I write, but Croatian makes this distinction much more clearly.

Yes, it is natural.

A perfective present form like napišem often does not mean an action happening right now in progress. Instead, it can describe:

• repeated completed actions
• general patterns
• what happens on particular occasions

So često napišem means something like:

• I often write it
• I often end up writing it
• On many occasions, I write it down

The speaker is viewing each instance as a complete little act.

If you said često pišem, that would also be possible, but it would sound a bit more like a general ongoing habit, with less focus on each completed occurrence.

radije means rather or preferably, and very often it is used where English would use prefer or would rather.

So:

• radije pišem = I prefer to write / I’d rather write

Croatian often expresses preference this way:

• Radije idem pješice. = I prefer to go on foot.
• Radije pijem čaj. = I prefer drinking tea.

There is also a verb preferirati, but radije is very common and often sounds more natural in everyday use.

Because adjectives must agree with the noun they describe in:

• gender
• number
• case

The noun here is e-mailu, which is:

• masculine
• singular
• locative

So the adjective must match it:

• nominative: službeni e-mail
• locative: u službenom e-mailu

That is why you get službenom.

Borrowed nouns are usually declined in Croatian too.

Here e-mail is treated as a masculine noun, so it changes by case:

• nominative: e-mail
• dative/locative: e-mailu

So:

• u e-mailu
• u službenom e-mailu

You may also see different spellings in real life, such as email or imejl, but the grammatical idea is the same.

It means the full expression or the whole phrase.

Here it contrasts with the abbreviation:

• kratica npr. = the abbreviation npr.
• cijeli izraz = the full phrase na primjer

So cijeli emphasizes that the speaker writes the expression out completely instead of shortening it.

No. Croatian word order is fairly flexible because case endings show the grammatical roles.

This version is natural:

• U poruci često napišem kraticu npr., ali u službenom e-mailu radije pišem cijeli izraz.

It begins with U poruci to set the context first, which is very common.

Other word orders are possible, but they slightly change emphasis. For example:

• Često u poruci napišem kraticu npr. puts a little more focus on often
• Kraticu npr. često napišem u poruci sounds more marked and emphasizes the object more strongly

So the original sentence has a very normal, smooth word order.

Not necessarily. poruka is a general word for message.

Depending on context, it can mean:

• a text message
• a chat message
• a direct message
• a message in a broader sense

In this sentence, it probably suggests a more informal kind of message, especially because it is contrasted with službeni e-mail, a formal email.