Se la coda alla cassa è troppo lunga, torno domani mattina.

Se means if. It introduces a condition:

Se la coda alla cassa è troppo lunga = If the line at the checkout is too long

This is a very common pattern in Italian: Se + present tense, present/future idea in the main clause

For example: Se piove, resto a casa. = If it rains, I stay / I’ll stay home.

It does literally mean tail, but in everyday Italian coda also means queue, line, or wait in line.

So here: la coda = the line / the queue

This is similar to how English can use one word in more than one sense. In context, alla cassa makes it clear that coda means a line of people waiting.

Yes, often you can. Both coda and fila can mean line/queue.

But there is a slight nuance:

• coda often emphasizes the idea of waiting in a queue
• fila can mean a line more generally, including a row of things or people

So: la coda alla cassa and la fila alla cassa are both natural.

In many everyday situations, native speakers use both.

Alla cassa means at the checkout, at the cash register, or at the cashier’s desk, depending on the context.

It is made of:

• a = at / to
• la cassa = the cash register / checkout

These combine into alla:

• a + la = alla

So: alla cassa = at the checkout

Because Italian usually uses a preposition where English might use a noun more directly.

English says:

• the line at the checkout

Italian says:

• la coda alla cassa

The preposition a helps show location: the queue is at the checkout.

Because coda is a feminine singular noun, so the adjective must agree with it.

• coda = feminine singular
• lunga = feminine singular form of lungo = long

Also:

• troppo = too
• è troppo lunga = is too long

Compare:

• Il film è troppo lungo. = masculine singular
• La coda è troppo lunga. = feminine singular

Because this is a normal, real condition, so Italian uses the indicative, not the subjunctive.

• Se la coda è troppo lunga... = If the line is too long...

The subjunctive is not used here just because of se. In fact, after se in ordinary if-clauses, Italian usually uses the indicative.

Italian often uses the present tense to talk about the near future, especially when the time is already clear.

Here: torno domani mattina literally looks like I return tomorrow morning, but it means I’ll come back tomorrow morning.

Because domani mattina clearly places the action in the future, the present tense sounds completely natural.

You could also say: tornerò domani mattina and that would also be correct, but torno domani mattina is very common and natural.

Because Italian usually drops subject pronouns when they are not needed.

Torno already means I return / I’ll come back, because the verb ending -o shows the subject is I.

So:

• torno = I return / I’m coming back / I’ll come back

You could say io torno, but that usually adds emphasis, such as: Io torno domani mattina, non oggi. = I’m coming back tomorrow morning, not today.

In this sentence, torno is best understood as I come back or I’ll come back.

The verb tornare often means:

• to return
• to come back
• to go back

The best English choice depends on context. In a shopping situation, I’ll come back tomorrow morning sounds very natural.

Because domani mattina is the normal Italian way to say tomorrow morning.

Italian commonly uses time expressions without extra prepositions:

• domani mattina = tomorrow morning
• stasera = tonight / this evening
• lunedì pomeriggio = Monday afternoon

So domani mattina is the standard phrase here.

Yes. You can say:

Se la coda alla cassa è troppo lunga, torno domani mattina.

or

Torno domani mattina se la coda alla cassa è troppo lunga.

Both are correct. The first version puts the condition first, which is very common. When the se clause comes first, a comma is often used in writing.

It usually refers to a specific situation: If the line at the checkout is too long, I’ll come back tomorrow morning.

But grammatically, the structure can also be used for habitual situations, depending on context.

For example: Se la coda è lunga, vado via. could mean:

• If the line is long, I leave as a general habit or
• If the line is long, I’ll leave in the current situation

In your sentence, domani mattina makes it sound like a decision about a particular situation.

Yes, it sounds natural and idiomatic.

Everything in it is everyday Italian:

• se for if
• la coda alla cassa for the line at the checkout
• present tense torno for a future action
• domani mattina as a normal time expression

A native speaker could easily say this in a supermarket, shop, or similar situation.