Ako zaboraviš kupaći kostim, mogu ti posuditi svoj stari.

In Croatian, after ako (if), you normally do not use the future tense the way English often does in meaning.

So Croatian says:

• Ako zaboraviš... = If you forget...

Here zaboraviš is a present-tense form, but because it follows ako and comes from the perfective verb zaboraviti, it refers to a future completed action: if it happens that you forget it.

Using Ako ćeš zaboraviti... would sound unnatural in standard Croatian.

Zaboraviš is the 2nd person singular present form of zaboraviti.

So:

• zaboraviti = to forget
• zaboraviš = you forget / if you forget

The ending -iš tells you the subject is you (singular), so Croatian does not need the pronoun ti here.

Kupaći kostim is the normal expression for a swimsuit or bathing suit.

Literally:

• kupaći = bathing / swimming
• kostim = suit, outfit, costume

As a whole, it means swimsuit. Even though kostim can sometimes look like English costume, here the phrase is a standard everyday word for swimwear.

Ti here means to you.

It is the dative singular form of ti (you).

So:

• mogu posuditi = I can lend
• mogu ti posuditi = I can lend to you

Croatian often uses short unstressed pronouns like this, called clitics. In English we would usually translate it as you in I can lend you..., but grammatically it is really to you.

It can be confusing, because Croatian posuditi can appear in contexts that English expresses with either lend or borrow, depending on the structure.

In this sentence:

• mogu ti posuditi svoj stari
  = I can lend you my old one

Why lend here? Because ti marks the person receiving the item.

A useful pattern:

• posuditi nekome nešto = to lend someone something
• posuditi nešto od nekoga = to borrow something from someone

So in this sentence, the meaning is clearly lend.

Croatian uses svoj as a reflexive possessive pronoun, meaning one’s own.

Here the subject of mogu is understood as I, so:

• mogu ti posuditi svoj stari = I can lend you my own old one

Croatian strongly prefers svoj when the possessor is the same as the subject of the clause.

So this is more natural than:

• mogu ti posuditi moj stari

Using moj is possible in some contexts for emphasis or contrast, but svoj is the standard choice here.

Because Croatian, like English, can leave out a noun when it is obvious from context.

So:

• svoj stari literally means my old one

The omitted noun is kupaći kostim.

Full version:

• mogu ti posuditi svoj stari kupaći kostim

But repeating the whole noun phrase would sound less natural here. Croatian often does exactly what English does with one:

• I can lend you my old one.

Because the omitted noun is kupaći kostim, which is masculine singular inanimate, and here it is in the accusative case as the direct object of posuditi.

For masculine inanimate nouns, the accusative singular looks like the nominative singular.

So:

• stari kostim = old swimsuit
• posuditi stari kostim = to lend an old swimsuit

That is why you get:

• svoj stari

and not svog starog, which would be used for masculine animate nouns.

Because Croatian usually leaves subject pronouns out when the verb ending already shows who the subject is.

Here:

• zaboraviš already means you forget
• mogu already means I can

So Croatian does not need:

• Ako ti zaboraviš..., ja mogu...

Those pronouns would only be added for emphasis, contrast, or special tone.

Yes, Croatian word order is fairly flexible, but this order is very natural.

The key point is that ti is a clitic, and clitics usually appear near the beginning of their clause, often in the second position.

So:

• mogu ti posuditi svoj stari = very natural

Other orders may be possible for emphasis, but not all of them sound equally neutral.

For example, the clitic ti usually cannot be placed just anywhere. Croatian has more word-order freedom than English, but clitics still follow special rules.

Because the sentence begins with a conditional subordinate clause:

• Ako zaboraviš kupaći kostim = if you forget your swimsuit

Then comes the main clause:

• mogu ti posuditi svoj stari = I can lend you my old one

In Croatian, when a subordinate clause comes first, it is normally separated from the main clause with a comma.