Molim vas, provjerite iskaznicu prije nego što mi date potvrdu.

Molim vas literally means (I) ask you / please, and it’s the common polite way to say please when addressing someone formally or more than one person.
You can say just Molim, but it’s more like Excuse me? / Pardon? / Yes? (e.g., when responding to someone), or it can sound incomplete on its own.

Vas is the accusative form of vi (you, formal singular or plural). After molim in this structure, Croatian uses the object form: Molim vas = I ask you / Please.
So vi is the subject form (you), while vas is used when you is the object.

Provjerite is the imperative, 2nd person plural form of provjeriti (to check/verify).
Croatian uses the plural imperative for:

• talking to multiple people, or
• speaking formally/politely to one person (like using you in English, but grammatically plural).

You’d switch to te and the singular imperative:

• Molim te, provjeri iskaznicu prije nego što mi daš potvrdu.
  (te = you, informal object; provjeri/daš = singular forms)

Iskaznicu is accusative singular of iskaznica. It’s accusative because it’s the direct object of provjerite (check what? → the ID card).
Dictionary form: iskaznica
Accusative singular: iskaznicu

Iskaznica is a general word for an ID card / identification card / membership card, depending on context.
For a personal ID card specifically, you’ll often see osobna iskaznica (personal ID card).

Prije nego što means before (…happens) and introduces a full clause with a verb:

• prije nego što mi date potvrdu = before you give me the confirmation

It’s different from prije + noun (e.g., prije ručka = before lunch), which doesn’t need a verb.

In standard Croatian, the common connector is prije nego što. Think of it as a fixed phrase meaning before + a clause.
You may also see slightly shorter variants like prije nego in some contexts, but prije nego što is very common and safe.

Mi is an unstressed clitic pronoun (dative to me), and Croatian clitics usually appear early in the clause—often right after the conjunction/introducer.
So što mi date is the natural placement: before you give me…

Mi is dative singular of ja (I), meaning to me.
date mi = give (to) me.

Both come from dati (to give), but:

• dajte = imperative (Give!)
• date = present tense (you give / you will give, depending on context)

After prije nego što, Croatian typically uses a normal verb form (often present), not an imperative. So prije nego što mi date… is the standard pattern.

Potvrdu is accusative singular of potvrda (confirmation / certificate / receipt). It’s accusative because it’s the direct object of date (give what? → the confirmation).
Dictionary form: potvrda
Accusative singular: potvrdu