Giulia indossa la maschera e guarda il fondo della piscina.

Indossare means to wear or to have on. In some contexts it can also be translated as to put on.

So Giulia indossa la maschera could mean:

• Giulia is wearing the mask
• Giulia puts on the mask

The most natural English choice depends on the situation being described.

In Italian, singular count nouns usually need an article.

So:

• la maschera = the mask
• una maschera = a mask

Saying just maschera by itself would normally sound incomplete in a sentence like this.

Because Giulia is a proper name, and in standard Italian proper names usually do not take an article.

So:

• Giulia indossa... = normal
• La Giulia indossa... = not standard in most neutral contexts, though articles with names can appear in some regional varieties or for special stylistic reasons

By itself, maschera can mean many kinds of mask.

But in the sentence Giulia indossa la maschera e guarda il fondo della piscina, the pool context strongly suggests a swimming mask, diving mask, or snorkeling mask rather than a carnival or theater mask.

Context tells you which kind of mask is meant.

Because guardare in Italian normally takes a direct object.

So Italian says:

• guardare qualcosa = to look at something / to watch something

Examples:

• Guarda il cielo = She looks at the sky
• Guardo la TV = I watch TV

English often needs at after look, but Italian usually does not use a preposition with guardare.

Yes.

Guardare can mean:

• to look at
• to watch

The object tells you which English translation sounds best.

Here:

• guarda il fondo della piscina is most naturally looks at the bottom of the pool

If the object were a film or a match, watches would be more natural:

• guarda un film = she watches a movie

Literally, it means the bottom of the pool.

Breakdown:

• il fondo = the bottom
• della piscina = of the pool

So fondo here means the bottom surface or floor of the pool.

Della is a contraction of:

• di = of
• la = the

So:

• di + la = della

Because piscina is a feminine singular noun, Italian uses la piscina, and therefore of the pool becomes della piscina.

Because Italian often uses an article where English may not seem to emphasize one.

Here, della piscina means of the pool, referring to a specific pool in the scene.

Compare:

• il fondo della piscina = the bottom of the pool
• il fondo di una piscina = the bottom of a pool

Using della is the normal, natural choice here.

Because both verbs have the same subject, so Italian does not need to repeat it.

• Giulia indossa la maschera e guarda il fondo della piscina

This works exactly like English:

• Giulia puts on the mask and looks at the bottom of the pool

You could repeat the subject, but it would usually be unnecessary:

• Giulia indossa la maschera e Giulia guarda... = grammatical, but unnatural unless you want special emphasis

Both are in the present indicative.

They are third person singular forms:

• indossa = she wears / puts on
• guarda = she looks at / watches

Because the subject is Giulia, English uses she.

This is the normal, neutral Italian word order:

• Giulia = subject
• indossa la maschera = first verb + object
• e guarda il fondo della piscina = second verb + object

So the pattern is basically subject + verb + object.

Italian word order can be more flexible than English, but this sentence is straightforward and standard.

From the articles:

• la maschera → maschera is feminine singular
• il fondo → fondo is masculine singular
• la piscina → piscina is feminine singular, which is why it becomes della piscina

The articles are your best clue:

• il = masculine singular
• la = feminine singular
• della = di + la, so feminine singular again

Yes. Italian is a pro-drop language, which means the subject pronoun or subject noun can often be omitted when it is already clear.

So if the context already made the subject obvious, Italian could say:

• Indossa la maschera e guarda il fondo della piscina.

That would mean:

• She wears/puts on the mask and looks at the bottom of the pool.

In your sentence, Giulia is included to make the subject explicit.