Martina mette il costume nello zaino prima di andare in piscina.

Here, costume means swimsuit, not a theater or Halloween costume.

In Italian, costume is often short for costume da bagno. Because the sentence mentions andare in piscina, the meaning swimsuit is the natural one.

So for English speakers, costume is a bit of a false friend in this context.

Because costume is a masculine noun in Italian.

Grammatical gender and real-life gender are not the same thing. Even if Martina is female, the noun costume is still masculine, so it takes il:

• il costume
• un costume

This does not mean the swimsuit belongs to a man.

Italian often uses the definite article when ownership is already clear from context.

Here, since the sentence is about Martina getting ready to go to the pool, it is easy to understand that it is probably her swimsuit, so il costume is enough.

You can say il suo costume if you want to make ownership more explicit or contrast it with someone else’s swimsuit.

Mette comes from the infinitive mettere, which means to put, to place, or sometimes to pack depending on context.

Mette is the 3rd person singular present indicative:

• io metto
• tu metti
• lui/lei mette

So Martina mette... means Martina puts... or Martina is putting..., depending on context.

It can mean either one.

Italian present tense often covers both:

• simple present: puts
• present progressive: is putting

The exact English translation depends on context. In this sentence, English might say:

• Martina puts the swimsuit in her backpack before going to the pool
• Martina is putting the swimsuit in her backpack before going to the pool

Sometimes packs is also a natural translation here.

Because zaino takes the article lo, not il.

That is because zaino begins with z. In Italian, masculine singular nouns starting with z often use lo:

• lo zaino
• lo studente
• lo psicologo

When in combines with lo, it becomes nello:

• in + lo = nello

So:

• nello zaino = in the backpack

Because Italian uses prima di + infinitive when the same person does both actions.

Here, Martina does both things:

1. she puts the swimsuit in the backpack
2. she goes to the pool

So Italian says:

• prima di andare in piscina = before going to the pool

This structure is very common:

• prima di mangiare
• prima di uscire
• prima di dormire

After prima di, Italian normally uses the infinitive if the subject is the same.

So:

• prima di andare = correct

Not:

• prima di va = incorrect

If the subject were different, Italian would usually use prima che plus a conjugated verb, often in the subjunctive.

For example:

• Prima che Martina vada in piscina, ...

But in your sentence, the same subject is understood, so prima di andare is the natural structure.

Because andare in piscina is the usual, idiomatic way to say to go to the pool or to go swimming.

In this context, in piscina is the standard choice. It sounds natural when talking about the activity or the place where you swim.

English speakers often want a direct equivalent of to the pool, but Italian does not always match English prepositions exactly. Here, you should learn andare in piscina as a common expression.

Because Italian often drops subject pronouns when the subject is already clear.

Here, the subject is already given as Martina, so adding lei is unnecessary.

Italian does this very often:

• Martina mette...
• not usually Martina lei mette...

The verb ending and the context normally give enough information.