Se dimentichi il costume, te ne presto uno pulito.

In Italian, a real future condition is very often expressed with the present tense in the se clause:

• Se dimentichi il costume... = If you forget the swimsuit...

This does not mean only a habitual action. It can also refer to a future possibility.

Italian often does this pattern:

• Se vieni, ti aiuto. = If you come, I’ll help you.
• Se piove, restiamo a casa. = If it rains, we’ll stay home.

So se dimentichi is completely natural here.

Here, costume most likely means swimsuit or bathing suit.

In everyday Italian, especially in context, costume is often short for costume da bagno.

So:

• il costume = the swimsuit / your swimsuit (depending on context)

Be careful: costume can also mean costume in English in other contexts, but in a sentence like this, it usually means swimsuit.

Italian often uses the definite article where English would use a possessive like your, especially when it is already obvious whose thing is being discussed.

So:

• Se dimentichi il costume...
  can naturally mean
• If you forget your swimsuit...

Italian does this a lot with clothing, body parts, and personal items when the owner is clear from context.

English usually prefers the possessive; Italian often does not need it.

Te ne is a combination of two pronouns:

• te = to you
• ne = of it / one of them / some of it, depending on context

In this sentence, ne refers back to costume.

So:

• te ne presto uno pulito

literally works like:

• I lend to you of-it one clean

Natural English:

• I’ll lend you a clean one

The ne is there because uno stands for one swimsuit, instead of repeating costume.

When certain Italian pronouns combine with ne, their form changes:

• mi → me
• ti → te
• si → se
• ci → ce
• vi → ve

So:

• ti + ne becomes te ne

That is why you get:

• te ne presto uno

not:

• ti ne presto uno

This is a fixed pronoun pattern in Italian.

No. Here presto is the verb prestare in the first person singular present:

• presto = I lend

So:

• te ne presto uno pulito = I’ll lend you a clean one

This is different from the adverb presto meaning soon.

Compare:

• Ti presto un libro. = I lend you a book.
• Arrivo presto. = I arrive soon / I’ll be there soon.

Same spelling, different meaning.

Because uno here is a pronoun, meaning one.

It replaces the noun costume:

• un costume pulito = a clean swimsuit
• uno pulito = a clean one

Use un when a noun follows:

• un costume

Use uno when the noun is omitted and one stands by itself:

• uno pulito

So uno pulito is exactly like English a clean one.

Because uno is standing in for the noun, and pulito describes that understood noun.

So:

• uno pulito = a clean one

This is the normal order in Italian.

You can think of it as short for:

• un costume pulito

Once costume is omitted, pulito still stays after the word standing in for it:

• uno pulito

Because Italian usually drops subject pronouns when the verb ending already makes the subject clear.

• presto already means I lend

So io is unnecessary unless you want emphasis or contrast.

Compare:

• Te ne presto uno pulito. = neutral
• Io te ne presto uno pulito. = I’ll lend you a clean one (with emphasis, maybe contrasting with someone else)

Italian often uses the present tense to talk about the near future, especially in everyday speech.

So:

• te ne presto uno pulito

can mean:

• I lend you a clean one
• I’ll lend you a clean one

Both are understood from context.

You could also say:

• Se dimentichi il costume, te ne presterò uno pulito.

That is also correct, but the present tense sounds very natural and conversational.

Yes. A more explicit version would be:

• Se dimentichi il costume, ti presto un costume pulito.

This is grammatically fine, but it repeats costume.

The original sentence:

• Se dimentichi il costume, te ne presto uno pulito.

sounds more natural because it avoids repetition and uses pronouns the way Italian often does.

So the original is more idiomatic.

It is informal, because it uses te as part of the pronoun combination and implies the tu form:

• dimentichi = you forget (informal singular)

If you were speaking formally to one person, you would say:

• Se dimentica il costume, gliene presto uno pulito.

That uses the formal Lei style.

So the original sentence is for talking to a friend, child, family member, or someone you address with tu.