Learn Hardcore Croatian: Kad poštar donese pismo iz knjižare, odmah ga otvaram i čitam što piše. - When the postman brings a letter from the bookstore, I immediately open it and read what it says.

In Croatian, after time conjunctions like kad / kada (when), čim (as soon as), dok (while), you normally use the present tense, even when you are talking about the future.

So:

Kad poštar donese pismo, otvorit ću ga.
= When the postman brings the letter, I will open it.

You do not say:
✗ Kad poštar će donijeti pismo…

In your sentence, the present tense donese can express:

a habitual action: Whenever the postman brings a letter…
or a future action (depending on context): When the postman brings the letter (later)…

English tends to use will or simple present in these clauses; Croatian just uses the present.

2. Why is it donese (from donijeti, perfective) and not donosi (from donositi, imperfective)?

Croatian has a strong aspect system: most verbs come in imperfective and perfective pairs.

donositi – imperfective: to be bringing, to bring regularly / in progress
(present: donosi)
donijeti – perfective: to bring (as one complete act)
(present: donese)

We use the perfective when we care about the action as a single completed event:

Kad poštar donese pismo…
When the postman (successfully) brings / has brought the letter…
→ completion of the bringing is important: only after that do you open it.

If you said:

Kad poštar donosi pismo…

it would sound odd here, as if you’re focusing on the ongoing process of bringing, not the completed arrival. For this “trigger” kind of clause (“once he has brought it, I do X”), the perfective donese is natural.

3. Why is there no ću in the first part, like kad će poštar donijeti pismo?

After kad / kada, Croatian does not normally use ću + infinitive for the future. You almost always use the present tense instead.

So:

Kad poštar donese pismo, odmah ga otvaram. ✅
✗ Kad će poštar donijeti pismo, odmah ga otvaram. ❌ (ungrammatical in standard Croatian)

If you want a clear future meaning, you put the future in the main clause, not in the kad-clause:

Kad poštar donese pismo, odmah ću ga otvoriti.
When the postman brings the letter, I will open it immediately.

4. What is the difference between kad and kada?

kad and kada mean the same thing: when.

kad – shorter, more informal, very common in speech
kada – slightly more formal or “complete”, common in writing and careful speech

In your sentence, both are fine:

Kad poštar donese pismo…
Kada poštar donese pismo…

There is no change in meaning; it’s just a stylistic choice.

5. Why is the pronoun ga used for pismo, which is neuter?

In Croatian, the unstressed object pronoun ga is used for:

masculine singular, accusative (e.g. vidim ga – I see him/it)
neuter singular, accusative, when it replaces a neuter noun

pismo (letter) is:

pismo – neuter singular, nominative
pismo – neuter singular, accusative (same form)

The pronoun that replaces it in the accusative is ga:

Otvaram pismo. → Otvaram ga.
I open the letter. → I open it.

So ga here stands for pismo, even though ga is also used for masculine. Context tells you which noun it refers to.

6. Why is the word order odmah ga otvaram, and not odmah otvaram ga?

Croatian has a fairly strict rule about where clitic pronouns (short unstressed forms like ga, je, mi, ti, se, li) go in the sentence. They usually come in second position (after the first stressed word or phrase).

In your clause:

Odmah – first stressed word
ga – clitic pronoun, goes right after odmah
otvaram – main verb

So:

Odmah ga otvaram. ✅ (natural, correct)
✗ Odmah otvaram ga. – sounds unnatural; it violates the usual clitic position rule.

You could also say:

Ja ga odmah otvaram. (if you really want to stress ja)
Here ja is first; ga is second; odmah otvaram follow.

7. Why is it iz knjižare and not od knjižare?

Both iz and od can sometimes translate as from, but they are used differently.

iz
- genitive → from the inside of a place / institution
  (iz kuće, iz škole, iz pošte, iz knjižare)
od
- genitive → from a person (or sometimes an abstract source)
  (pismo od mame, od prijatelja, od šefa)

In pismo iz knjižare:

iz knjižare literally means from the bookstore, understood as an institution / place that sent the letter.

You could say pismo od knjižare, but it sounds more like “a letter from the bookstore (as a sender)” where you think of the bookstore as a somewhat personified source. iz knjižare is very natural when thinking of it as coming out of that place.

8. What exactly does pismo iz knjižare mean? Is it “a letter of the bookstore” or “a letter from the bookstore”?

pismo iz knjižare means “a letter from the bookstore” (sent by / coming from the bookstore).

pismo – letter
iz – from (out of)
knjižare – bookstore (genitive singular of knjižara)

So literally: a letter from (out of) the bookstore.
It does not mean “letter of the bookstore” in the sense of possession; it’s about origin / source.

Compare:

pismo iz Pariza – a letter from Paris
pismo od brata – a letter from (my) brother

9. What does čitam što piše literally mean, and why does it translate as “I read what it says”?

Word by word:

čitam – I read
što – what
piše – writes / is written / it says

In Croatian, pisati (to write) in the 3rd person singular piše is often used impersonally to mean:

što piše – what is written / what it says (in the letter, in the book, on the sign)

So:

čitam što piše ≈ I read what is written (in it)
or more idiomatically: I read what it says.

Fuller versions are also common:

čitam što piše u pismu – I read what it says in the letter.
čitam što piše unutra – I read what it says inside.

Your sentence just omits u pismu because it’s obvious from context.

10. Why is it što and not šta?

Both što and šta mean what.

što – standard, used in formal language, writing, and in many regions
šta – widespread in speech, especially in Bosnia, Serbia, and parts of Croatia (Dalmatia, etc.); considered more colloquial / dialectal in standard Croatian

In a standard sentence like yours, što is the neutral choice:

čitam što piše – standard
čitam šta piše – commonly heard in speech in many areas, but less formal

11. Could I say čitam ono što piše or čitam što on piše? What’s the difference?

Yes, you can, but the meaning shifts slightly.

čitam ono što piše
- ono = that (thing)
- Literally: I read that which is written / I read what is written.
- Slightly more explicit / emphatic than just čitam što piše, but very similar in meaning.
čitam što on piše
- on = he
- Literally: I read what he writes.
- Now you are focusing on a person as the writer (the postman? the bookstore? some author?), not just “what is written in the letter.”

Your original čitam što piše is neutral and focuses on the content of the letter, not on who wrote it.

12. Why is there a comma after knjižare but not before što piše?

The sentence is divided like this:

Kad poštar donese pismo iz knjižare, → subordinate time clause
odmah ga otvaram i čitam što piše. → main clause + object clause

Rules involved:

You do put a comma between a subordinate clause and the main clause:
- Kad poštar donese pismo iz knjižare, odmah ga otvaram…
You normally do not put a comma before što when it introduces an object clause tightly connected to the verb:
- čitam što piše – I read what it says
  (No comma: čitam, što piše would be wrong here.)

So the comma is only marking the boundary between the time clause and the main clause, not between čitam and što piše.