Learn Hardcore Croatian: U banci se čeka u redu. - At the bank one waits in a line.

In U banci se čeka u redu, se forms an impersonal/reflexive-passive construction. It means “one waits/people wait/you wait” in a general, rule-like way. There is no explicit subject. So the sentence means “At the bank, you wait in line” (generally).

Can I drop se and just say U banci čeka u redu?

No. Without se, the verb needs a subject, so U banci čeka u redu is ungrammatical. You have two good options:

Keep the impersonal: U banci se čeka (u redu).
Use an explicit subject: Ljudi u banci čekaju u redu.

Why is it u banci and not u banku or u banka?

Because u takes:

Locative for a static location: u banci = “in/at the bank”
Accusative for motion into: u banku = “into the bank” There are no articles in Croatian, so context supplies “the/a.”

Why is it banci and not banki?

It’s a spelling/phonological change (palatalization): in the dative/locative singular, k + i often becomes c + i. So from banka (bank), you get u banci (in the bank), not “banki.”

What exactly does u redu mean here? Doesn’t u redu also mean “OK”?

Here u redu means “in (a) line/queue,” because red is the noun “line/row/queue.”
Separately, u redu is also a common idiom meaning “OK/All right,” as in Sve je u redu (“Everything’s OK”). Context tells them apart.

Could I say na redu or čekati red instead?

Different meanings:

čekati u redu = “wait in (the) line/queue” (physical queue)
čekati red = “wait for one’s turn”
biti na redu = “to be next (it’s your turn),” e.g., Ja sam na redu (“I’m next”)

Why is se after u banci and not next to the verb?

Se is a clitic and wants “second position” in the sentence (after the first stressed word/phrase). Since the sentence starts with U banci, se follows it: U banci se čeka…
You can also say Čeka se u banci, where se still sits in second position.

Is there a difference between čeka se and stoji se u redu?

čeka se (u redu) puts the focus on the act of waiting.
stoji se u redu emphasizes physically standing in line.
Both are natural; čekati u redu is the more typical collocation for the idea of queuing.

Is the sentence generic (like “At banks, you wait in line”) or specific (“At the bank, you wait in line”)?

As written, it can be either, depending on context (Croatian has no articles). To make it clearly generic, use the plural: U bankama se čeka u redu (“In banks, people wait in line”).

Which case is redu, and is that the normal form after u?

Redu is locative singular of red (“line/row/queue”). After u with a static location, you use the locative: u redu (“in the queue”). With motion into a queue you’d still normally say u red, but that’s rare; idiomatically people say stati u red (“to get in line”).