Learn Hardcore Croatian: Kupac u poruci pita može li dobiti veći popust jer dostava kasni. - The customer asks in a message whether they can get a bigger discount because the delivery is late.

Kupac (customer/buyer) is the subject of the sentence, so it’s in the nominative case. Croatian often places the subject early, but word order is flexible; the meaning is mainly carried by cases and verb forms.

What does u poruci mean grammatically? Why u + this form?

U poruci = in the message. After u meaning location (in/inside), Croatian uses the locative case.

Base form: poruka (message)
Locative singular: poruci
So: u poruci.

Could I also say Kupac porukom pita... instead of Kupac u poruci pita...?

Yes, but it changes the nuance:

u poruci = in the message (as a place/context)
porukom (instrumental) = by message / via a message (the means)
Both are natural; u poruci emphasizes the content inside the message, while porukom emphasizes the communication channel.

Why is pita in the present tense—shouldn’t it be past?

Croatian often uses the present tense to describe what someone “says/asks” in a general, current, or narrative sense (especially in summaries). If you wanted past:

Kupac je u poruci pitao... = The customer asked in the message...

How does može li work? Why is li there?

Li is a question particle used in yes/no questions, especially in more formal or neutral phrasing.
može li literally means can (he) ... ? / is it possible ... ? and introduces an embedded yes/no question after pita (asks).

Where is the word for he? Why isn’t on included?

Croatian commonly drops subject pronouns because the verb form already implies the subject.
Here, može = (he/she/it) can. Context tells us it’s the customer. You can add on for emphasis or contrast, but it’s usually unnecessary.

Why is it može li dobiti (can he get) and not something like može li da dobije?

Standard Croatian typically uses moći + infinitive:

može li dobiti = can he get
The da + present construction is common in some neighboring standards/varieties, but it’s not the default in standard Croatian.

What case is veći popust, and why?

Veći popust is the direct object of dobiti (to get), so it’s in the accusative. For many masculine inanimate nouns, the accusative looks the same as the nominative:

popust (nom) = popust (acc)

Why is it veći? What form is that?

Veći is the comparative form of velik (big/large), meaning bigger/greater. It agrees with popust (masculine singular):

veći popust = a bigger discount

Could I say veći vs veću vs veće—how do I choose?

It depends on the gender (and case/number) of the noun:

masculine: veći popust
feminine: veću cijenu (accusative feminine) / veća cijena (nominative feminine)
neuter: veće selo
So you choose the adjective ending that matches the noun’s gender (and its case/number).

What does jer do, and is it different from zato što?

Jer means because and introduces the reason: because shipping is late.
Zato što also means because, often slightly more explicit/stronger. In many everyday contexts, they’re interchangeable:

... jer dostava kasni.
... zato što dostava kasni.

Why is it dostava kasni—what does kasni mean exactly?

Kasni means is late / is delayed (present tense of kasniti, to be late).
Dostava (delivery/shipping) is the subject (nominative), so:

dostava kasni = the delivery is late / shipping is delayed.