Se senti un altro tuono, chiudi la finestra e passa un fazzoletto a tua sorella: anche lei ha il raffreddore.

In Italian, after se when you are talking about a real, possible condition, the present tense is very common:

• Se senti un altro tuono, chiudi la finestra.
• literally: If you hear another thunderclap, close the window.

English often uses the present in the same kind of sentence too: If you hear..., not if you will hear...

So senti here is completely normal. It means if you hear.

Yes. Both are informal singular commands addressed to tu:

• chiudi = close
• passa = hand / pass

So the speaker is talking to one person in an informal way.

For comparison:

• (tu) chiudi = close
• (Lei) chiuda = close (formal)
• (voi) chiudete = close (plural)

The sentence is basically: If you hear another thunderclap, close the window and hand a tissue to your sister.

Tuono means thunder or more precisely a thunderclap / rumble of thunder.

So un altro tuono means:

• another clap of thunder
• another rumble of thunder

In context, it refers to hearing thunder again.

Italian often uses the definite article where English might say the or sometimes just understand it from context.

Here chiudi la finestra means close the window, probably the window that is already relevant in the situation.

If you said chiudi una finestra, it would sound more like close a window, any window, which is less natural in this context.

Here passare means to hand, to pass, or to give across.

So:

• passa un fazzoletto a tua sorella = hand a handkerchief / tissue to your sister

A native English speaker might first think of passa as only meaning passes in a movement sense, but in Italian it is also used for physically passing an object to someone.

Literally, fazzoletto traditionally means handkerchief.
But depending on context, learners may also understand it as a small cloth or something used for the nose.

In modern everyday speech, if someone specifically means a disposable paper tissue, they very often say:

• un fazzoletto di carta

In your sentence, fazzoletto works because the context is clear: the sister has a cold.

Because a marks the indirect object: the person receiving the thing.

• passa un fazzoletto a tua sorella
• literally: pass a tissue to your sister

This is like English to your sister.

Compare:

• dare qualcosa a qualcuno = to give something to someone
• passare qualcosa a qualcuno = to pass something to someone

With singular family members, Italian usually drops the article before the possessive:

• mia madre
• tuo fratello
• tua sorella

So a tua sorella is the normal form.

You would normally use the article with:

1. Plural family members

   • ai tuoi fratelli
   • le mie sorelle

2. Some modified forms or special cases

   • la tua sorella maggiore
   • il mio caro fratello

So in this sentence, a tua sorella is exactly what you would expect.

Anche lei means she too / she also.

The word order is natural because anche comes before the element it is focusing on:

• anche lei = she too
• anche io = me too
• anche tu = you too

Here it means that the sister also has a cold, probably like someone else already mentioned or implied.

In Italian, many illnesses are expressed with avere + article + noun:

• avere il raffreddore = to have a cold
• avere la febbre = to have a fever
• avere l’influenza = to have the flu

So anche lei ha il raffreddore means she has a cold too.

Using the article here is normal Italian grammar.

The colon introduces an explanation or reason for what came before.

So:

• Se senti un altro tuono, chiudi la finestra e passa un fazzoletto a tua sorella: anche lei ha il raffreddore.

The colon signals something like:

• ...because she has a cold too
• ...after all, she has a cold too

It connects the command about the tissue to the explanation that follows.

By itself, passa can be:

• he/she/it passes
• or the tu imperative of passare

But in this sentence, the context makes it clearly a command:

• Se senti..., chiudi..., e passa...

You have a conditional clause followed by two instructions, so passa is understood as hand/pass.

Yes. Italian often leaves subject pronouns out when the verb ending already shows who the subject is.

For example:

• senti already tells you it is tu
• chiudi also matches tu
• passa here is the tu command

So Italian normally says:

• Se senti..., chiudi...

rather than:

• Se tu senti..., tu chiudi...

The pronoun tu would only be added for emphasis or contrast.