Ako vidiš labuda, pokaži mi ga.

Croatian has no articles. There is no separate word for a or the.

So labuda can mean a swan or the swan, depending on context. In this sentence, English naturally translates it as a swan, but Croatian does not mark that distinction with an article.

Because labuda is the accusative singular form of labud.

Here, labud is the direct object of vidiš (see), so it must go into the accusative case.

A useful rule:

• masculine animate nouns often have
  • nominative: labud
  • accusative: labuda

Since a swan is an animate being, the accusative changes form.

Croatian usually drops subject pronouns when they are clear from the verb ending.

• vidiš = you see
• pokaži = show! addressed to one person

So ti (you) is not necessary here. You could add ti for emphasis, but the neutral sentence normally leaves it out.

Vidiš is the 2nd person singular present tense form of vidjeti (to see).

So:

• ja vidim = I see
• ti vidiš = you see

In this sentence, it appears after ako (if), and Croatian normally uses the present tense in this kind of clause:

• Ako vidiš... = If you see...

Croatian would normally not use a future form here.

Because after ako (if), Croatian commonly uses the present tense to talk about a possible future situation.

So:

• Ako vidiš labuda... literally looks like If you see a swan...
• but it naturally refers to a future possibility: If you happen to see a swan...

This is normal Croatian grammar. It is similar to English, which also says If you see..., not If you will see...

Pokaži is the imperative form, singular, from pokazati (to show).

It means:

• show! when speaking to one person

So:

• pokaži = show!
• pokažite = show! (to more than one person, or formal singular)

Mi is the short, unstressed clitic form of meni (to me).

So:

• meni = full/stressed form
• mi = short/neutral form

In ordinary sentences, Croatian usually prefers the short form:

• Pokaži mi ga. = Show it to me.

You would use meni when you want emphasis or contrast:

• Pokaži meni, ne njemu. = Show me, not him.

Ga is the short pronoun meaning him/it, here referring back to labuda.

Since labud is masculine singular, the pronoun that replaces it is ga in this structure:

• vidiš labuda = you see a swan
• pokaži mi ga = show it to me

Even though English uses it for an animal here, Croatian chooses the pronoun based on the noun’s grammatical gender, so ga is the correct form.

Croatian short pronouns (clitics) follow a fairly fixed order.

In a neutral sentence, the order here is:

• mi = to me
• ga = it/him

So:

• pokaži mi ga = the normal order

The sequence ga mi would sound wrong in standard neutral Croatian.

Because pokaži mi ga is the most neutral, natural way to say it.

Croatian usually places short unstressed pronouns right after the first stressed element of the clause, and they come in a set order. That gives:

• pokaži mi ga

You can say pokaži ga meni, but that changes the feel:

• meni is stressed
• it sounds more emphatic or contrastive

So the given sentence is the normal everyday version.

Because Ako vidiš labuda is a subordinate if-clause, and it comes before the main clause:

• Ako vidiš labuda, = If you see a swan,
• pokaži mi ga. = show it to me.

In Croatian, when this kind of subordinate clause comes first, it is normally separated by a comma.

Yes, but the meaning changes slightly.

• ako = if
  This sounds conditional: maybe it happens, maybe it does not.
• kad = when
  This sounds more like the situation is expected, or like whenever

So:

• Ako vidiš labuda, pokaži mi ga. = If you see a swan, show it to me.
• Kad vidiš labuda, pokaži mi ga. = When you see a swan, show it to me.

Both are grammatical, but ako matches the English if more closely.