Learn Hardcore Croatian: Pošta je malo dalje u istoj ulici, ali u pošti obično nema velikog reda. - The post office is a bit further on the same street, but in the post office there is usually no big line.

Pošta is in the nominative singular in Pošta je malo dalje… because it’s the subject of the sentence: The post office is…
Pošti is in the locative singular in u pošti obično nema… because the preposition u (in/at) requires the locative when it means a static location: in/at the post office.

So:

pošta = nominative (subject)
u pošti = in/at the post office (locative after u)

Why is it u istoj ulici and not something like na istoj ulici, when English says on the same street?

Croatian prepositions don’t always match English ones.

In Croatian, you normally say u ulici (literally in the street) for addresses and locations along a street.
u istoj ulici = in the same street, but it corresponds to English on the same street.

So:

u
- locative (istoj ulici) is the standard expression here.
na ulici would usually suggest being physically out in the street, like on the street (outside), not just located along that street by address.

What exactly does malo dalje mean, and why are there two words?

malo dalje literally means a little further.

malo = a little, a bit (adverb)
dalje = further (the comparative form of daleko = far)

Put together:

Pošta je malo dalje… = The post office is a bit further (away)…

Both words are adverbs: malo modifies dalje and makes it softer, just like a bit modifies further in English.

Why is the verb je in the second position: Pošta je malo dalje and not Je pošta malo dalje?

Je is a clitic form of the verb biti (to be), 3rd person singular: (ona) je = it is / she is.

In Croatian, clitics (short, unstressed forms like je, se, ga, mi, ti) prefer the second position in the sentence or clause (the so-called Wackernagel position):

Pošta je malo dalje… – subject first (Pošta), then clitic verb (je), then the rest.

Starting a sentence with Je pošta… is ungrammatical in standard Croatian. You can move the subject around, but je will still try to be in the second slot:

Tamo je pošta.
Pošta je tamo.
but not Je tamo pošta.

Why is it u istoj ulici and not just u ista ulica?

Adjectives and nouns must agree in gender, number, and case.

ulica (street) is feminine, singular.
In u istoj ulici, the case is locative (after u with static location).

So both words take the feminine singular locative form:

ista (same) → istoj
ulica → ulici

Hence:

u istoj ulici = in the same street
not u ista ulica, which would be nominative and ungrammatical here.

What is the difference between pošta as a building and post/mail as a service? Does pošta cover both?

Yes, pošta can mean both:

The post office (building):
- Pošta je malo dalje u istoj ulici.
  = The post office is a little further down the same street.
The postal service / mail in general:
- Danas pošta kasni.
  = The mail is late today.

Context tells you which meaning is intended. In this sentence, because we’re talking about a physical location u istoj ulici, it clearly means the post office (building).

In u pošti obično nema velikog reda, why is it nema and not nije?

Croatian often uses the verb imati (ima / nema) to express there is / there isn’t:

Ima reda. = There is a line / queue.
Nema reda. = There is no line / There isn’t a line.

So:

u pošti obično nema velikog reda
literally: in the post office usually there-is-not a big line.

Using nije would require a different structure:

Red nije velik. = The line is not big.

But here the idea is there isn’t a big line, so nema is the natural choice.

Why is it velikog reda instead of veliki red after nema?

After nema (there is not / there isn’t), Croatian normally uses the genitive case to express the absence of something.

veliki red = big queue/line (nominative, as subject)
velikog reda = of a big queue/line (genitive)

Because nema takes the genitive:

Nema reda. = There is no line.
Nema velikog reda. = There is no big line / There usually isn’t a big line.

So:

red → genitive singular: reda
veliki → genitive masculine singular: velikog

That’s why it’s velikog reda after nema.

How does obično work in this sentence? Can it be placed somewhere else?

Obično is an adverb meaning usually / generally / typically.

In u pošti obično nema velikog reda it modifies the whole predicate nema velikog reda:

u pošti – in the post office
obično – usually
nema velikog reda – there isn’t a big line

You can move obično to other positions without changing the meaning much:

Obično u pošti nema velikog reda.
U pošti nema obično velikog reda. (possible, but less common / slightly marked)

The most neutral, natural positions are:

U pošti obično nema velikog reda.
Obično u pošti nema velikog reda.

Could you also say Pošta je malo dalje niz ulicu? Would that sound natural?

Yes, you can say that, and it’s natural, but with a slightly different nuance:

Pošta je malo dalje u istoj ulici.
= The post office is a bit further along the same street. (more neutral)
Pošta je malo dalje niz ulicu.
literally = The post office is a bit further down the street.

Niz ulicu slightly emphasizes the direction down/along the street, often in a more physical, spatial sense.
U istoj ulici simply states that it’s located somewhere further on that same street, without that directional flavor.

Can the word order in the second clause be changed, like Obično u pošti nema velikog reda? Is that still correct?

Yes, the word order is flexible, and that version is also correct:

U pošti obično nema velikog reda.
Obično u pošti nema velikog reda.

Both are natural. The difference is very small:

Starting with Obično slightly emphasizes the habitual nature: Usually, in the post office, there isn’t a big line.
Starting with U pošti slightly emphasizes the location first: In the post office, there usually isn’t a big line.

Grammatically, both are fine; it’s mostly about focus and rhythm.