Se perdo la coincidenza, aspetto il prossimo treno.

In Italian, the subject pronoun is often omitted because the verb ending already shows who the subject is.

• perdo = I lose / if I miss
• aspetto = I wait

So Se perdo la coincidenza, aspetto il prossimo treno naturally means If I miss the connection, I wait for the next train without needing io.

You can add io for emphasis:

• Se io perdo la coincidenza, aspetto il prossimo treno

But in normal speech, that usually sounds unnecessary unless you want contrast, such as I do this, not someone else.

Italian often uses the present tense to talk about future situations when the meaning is clear from context, especially in if sentences.

So:

• Se perdo la coincidenza, aspetto il prossimo treno

can mean:

• If I miss the connection, I wait for the next train
• or more naturally in English, If I miss the connection, I’ll wait for the next train

This is very normal in Italian. English often prefers will in the main clause, but Italian commonly keeps the present:

• Se piove, resto a casa = If it rains, I’ll stay home

After se meaning if, Italian normally does not use the future tense for a real possible condition.

So Italian says:

• Se perdo la coincidenza...

not:

• Se perderò la coincidenza...

This is different from English, where learners may be tempted to think in terms of future meaning.

A good basic pattern is:

• Se + present, present/future meaning
• If + present, future meaning in English

Examples:

• Se arrivo tardi, ti chiamo = If I arrive late, I’ll call you
• Se ho tempo, passo da te = If I have time, I’ll stop by

Because this sentence expresses a real, possible condition, not doubt or a wish.

With se meaning if, Italian usually uses:

• the indicative for real or likely conditions
• the imperfetto congiuntivo and conditional for more hypothetical or unreal situations

So here:

• Se perdo la coincidenza, aspetto il prossimo treno
  is a normal real condition.

Compare it with a more hypothetical sentence:

• Se perdessi la coincidenza, aspetterei il prossimo treno
  = If I missed the connection, I would wait for the next train

So perdo is correct because this is a straightforward, realistic possibility.

Not always. This is a very important vocabulary point.

In travel contexts, coincidenza means connection, as in a connecting train, bus, or flight.

So:

• perdere la coincidenza = to miss the connection

Italian coincidenza can also mean coincidence in the usual English sense, but in transport it has this specific meaning.

Examples:

• Ho perso la coincidenza per Roma = I missed my connection to Rome
• Che coincidenza! = What a coincidence!

So it is a word whose meaning depends strongly on context.

Here, la coincidenza refers to the specific connection relevant to the trip.

When you are traveling, there is usually one particular connecting train or connection you are talking about, so the definite article sounds natural:

• Se perdo la coincidenza... = If I miss the connection...

Using una coincidenza would sound less specific, more like a connection in general. That is grammatically possible in some contexts, but this sentence most naturally refers to a known, specific one.

Here prossimo means next.

So:

• il prossimo treno = the next train

It agrees with treno, which is masculine singular:

• il prossimo treno
• la prossima coincidenza
• i prossimi treni
• le prossime coincidenze

A useful note: prossimo often comes before the noun in very common expressions like this:

• la prossima volta = next time
• il prossimo anno = next year

Yes, that is grammatically possible.

Both of these are correct:

• Se perdo la coincidenza, aspetto il prossimo treno
• Aspetto il prossimo treno se perdo la coincidenza

The first version is often more natural because it presents the condition first and then the result. That is especially common in if-sentences.

Also, when the se clause comes first, a comma is commonly used:

• Se perdo la coincidenza, aspetto il prossimo treno

When the main clause comes first, the comma is usually omitted:

• Aspetto il prossimo treno se perdo la coincidenza

When the sentence starts with the se clause, the comma is standard and helpful:

• Se perdo la coincidenza, aspetto il prossimo treno

It marks the pause between the condition and the result.

If the order is reversed, Italian usually does not use the comma:

• Aspetto il prossimo treno se perdo la coincidenza

So in your original sentence, the comma is the normal choice.

Yes, and the meaning changes slightly.

• aspetto il prossimo treno = I wait for the next train
• prendo il prossimo treno = I take the next train

The original sentence focuses on the act of waiting after missing the connection.
If you want to emphasize what you do next as a travel decision, prendo il prossimo treno may sound more direct.

For example:

• Se perdo la coincidenza, aspetto il prossimo treno
  = I wait for the next train.
• Se perdo la coincidenza, prendo il prossimo treno
  = I take the next train.

Both are natural, but they highlight different parts of the situation.

Then you would change the tenses:

• Se perdessi la coincidenza, aspetterei il prossimo treno

This uses:

• perdessi = imperfect subjunctive
• aspetterei = conditional

This pattern is used for a more hypothetical, less direct, or unreal situation.

So compare:

• Se perdo la coincidenza, aspetto il prossimo treno
  = If I miss the connection, I’ll wait for the next train.
• Se perdessi la coincidenza, aspetterei il prossimo treno
  = If I missed the connection, I would wait for the next train.

That contrast is very useful in Italian.

Yes, it is a very common expression in travel Italian.

• perdere = to lose / miss
• perdere la coincidenza = to miss the connection

Similarly, Italian often uses perdere where English uses miss:

• perdere il treno = to miss the train
• perdere l’aereo = to miss the plane
• perdere l’autobus = to miss the bus

So this sentence uses a very standard travel phrase, not an unusual or literary one.