Molim te, ako vidiš onu omotnicu, donesi mi je prije nego što izađeš.

Literally, molim means I ask / I beg / I request, and te means you in the accusative case.

So grammatically, Molim te is something like I ask you, but in everyday Croatian it very often functions simply as please.

Why te? Because moliti normally takes a direct object: you are asking someone.

• ja molim tebe = I ask you
• short unstressed form: te instead of tebe

So Molim te is both idiomatic and completely grammatical.

Because Croatian usually drops subject pronouns when they are not needed.

The verb endings already show who the subject is:

• vidiš = you see
• izađeš = you go out / you leave

So adding ti is usually unnecessary unless you want emphasis or contrast:

• ako vidiš... = normal
• ako ti vidiš... = more emphatic, like if you see...

This is very common in Croatian.

Because both words are in the accusative singular, not the dictionary form.

The dictionary form is:

• omotnica = envelope nominative singular

But here omotnicu is the direct object of vidiš and also the thing being brought, so Croatian uses the accusative:

• vidiš omotnicu = you see the envelope
• donesi omotnicu = bring the envelope

The demonstrative must match too:

• nominative: ona omotnica
• accusative: onu omotnicu

So the ending changes because of case.

Onu is the feminine accusative singular of onaj, meaning that.

Croatian demonstratives roughly work like this:

• ovaj = this, near the speaker
• taj = that, near the listener / already mentioned
• onaj = that over there / more distant / more specifically singled out

In real life, the distinctions are not always rigid, but onu omotnicu strongly points to a particular envelope.

Compare:

• ako vidiš omotnicu = if you see an envelope / the envelope
• ako vidiš tu omotnicu = if you see that envelope
• ako vidiš onu omotnicu = if you see that particular envelope over there / the one I mean

So onu helps identify a specific object.

Here je means it.

More specifically, it is the unstressed accusative singular feminine pronoun, referring back to omotnicu.

Since omotnica is feminine, the pronoun must also be feminine:

• omotnica → je = it

So:

• donesi mi je = bring it to me

Important: this je is not the verb is here. Croatian je can be either:

• je = is
  or
• je = her / it (unstressed object pronoun)

In this sentence, it is the pronoun it.

Because Croatian unstressed pronouns, called clitics, follow a fairly strict order.

In a cluster like this, dative usually comes before accusative:

• mi = to me
• je = it

So the normal order is:

• donesi mi je = bring it to me

Not:

• donesi je mi ✗

Croatian clitic order is one of those things that learners usually need to get used to rather than translate word-for-word from English.

Yes, but it changes the tone.

• donesi mi je = normal, neutral, everyday
• donesi meni nju = stressed/emphatic, something like bring it to me or bring that one to me

The longer forms meni and nju are stressed pronouns. They are used when you want contrast, emphasis, correction, or emotional weight.

For example:

• Ne njemu — meni nju donesi. = Not to him — bring it to me.

So donesi meni nju is possible, but it is not the ordinary neutral version.

Because donesi is the imperative of a perfective verb, and it fits a single completed action: bring it.

The speaker wants one complete result:

• see the envelope
• bring it
• done

That is why donesi sounds natural.

By contrast, donosi would come from an imperfective verb and would sound more like:

• bring it repeatedly
• be in the process of bringing it
• keep bringing

That is not what this sentence is asking for. For one specific completed action, Croatian usually prefers the perfective imperative.

Yes. Prije nego što is a set phrase meaning before in the sense of before (someone) does something.

It introduces a subordinate clause:

• prije nego što izađeš = before you go out / before you leave

You can think of it as:

• prije = before
• nego što = than/that-part of the connector

But for learners, it is easiest to treat prije nego što as a single conjunction-like expression meaning before.

Because Croatian often uses the present form in subordinate clauses to refer to a future action, especially after words like:

• kad = when
• ako = if
• čim = as soon as
• prije nego što = before

So:

• prije nego što izađeš literally looks like before you go out
• but it naturally refers to a future event: before you leave

This is normal Croatian usage. English also does something similar:

• before you leave, not usually before you will leave

So this part is actually closer to English than it may first seem.

For the same reason: after ako (if), Croatian normally uses the present form, not a future auxiliary construction.

So:

• ako vidiš = if you see
• not usually ako ćeš vidjeti

This is the standard way to express a future condition in Croatian.

Also, in this sentence the meaning is conditional and natural:

• if you happen to see that envelope, bring it to me...

Yes, Croatian word order is more flexible than English word order, but it is not completely free.

This sentence could be rearranged somewhat for focus or style, for example:

• Ako vidiš onu omotnicu, molim te, donesi mi je prije nego što izađeš.

That is still natural.

But the clitics must behave properly:

• mi je should stay together in the correct clitic order
• donesi je mi is wrong

So the bigger chunks can move around more easily than the small unstressed pronouns.

The commas separate natural spoken chunks and the inserted conditional clause.

• Molim te, is set off like a polite parenthetical expression
• ako vidiš onu omotnicu, is a subordinate conditional clause

So the sentence is structured like this:

• Molim te,
• ako vidiš onu omotnicu,
• donesi mi je prije nego što izađeš.

The commas help show where those pieces begin and end.