Se non riesci ad aggiustare la cerniera, domani torniamo dalla sarta.

Because Italian often omits subject pronouns when the verb ending already shows who the subject is.

• riesci = you manage / you can
• torniamo = we return / we go back

So:

• Se non riesci... = If you can’t / don’t manage...
• domani torniamo... = tomorrow we’re going back...

You could add tu or noi, but it would usually be for emphasis:

• Se tu non riesci...
• Domani noi torniamo...

Most of the time, they are unnecessary.

Se means if.

It introduces a condition:

• Se non riesci ad aggiustare la cerniera...
• If you can’t fix the zipper...

So the whole sentence is a normal if-sentence: if one thing happens, then another thing will happen.

Riuscire a + infinitive means to manage to do something, to succeed in doing something, or sometimes simply to be able to do something.

So:

• riesci ad aggiustare = you manage to fix / you can fix

Examples:

• Non riesco a dormire. = I can’t manage to sleep.
• Sei riuscito a chiamarla? = Did you manage to call her?

It is a very common pattern in Italian:

• riuscire a fare
• riuscire a capire
• riuscire a trovare

Because riuscire is followed by a + infinitive:

• riuscire a fare qualcosa

Here the next word, aggiustare, begins with a vowel, so a often becomes ad for smoother pronunciation:

• riuscire ad aggiustare

So:

• riesci ad aggiustare = correct and natural

You need that preposition after riuscire.
You cannot say:

• riesci aggiustare ❌

Yes, both aggiustare and riparare can mean to fix / repair.

In everyday speech, aggiustare is very common for practical things:

• aggiustare la cerniera
• aggiustare la bici
• aggiustare il telefono

Riparare is also correct, and can sound a little more like repair in a general sense.

So both are possible:

• aggiustare la cerniera
• riparare la cerniera

In this sentence, aggiustare sounds very natural and conversational.

Italian uses definite articles much more often than English does.

So where English might say:

• fix the zipper or even in some contexts just
• fix zipper

Italian normally says:

• aggiustare la cerniera

The article la is expected here because you are talking about a specific zipper, probably the one on a particular item of clothing.

No. Cerniera can mean different things depending on context.

Common meanings include:

• zipper
• hinge

In this sentence, because of the seamstress (la sarta), the meaning is clearly zipper.

So:

• la cerniera della giacca = the zipper of the jacket
• la cerniera della porta = the door hinge

Context tells you which meaning is intended.

Italian often uses the present tense to talk about the near future, especially with a time word like domani (tomorrow).

So:

• domani torniamo dalla sarta
  can mean
• tomorrow we’re going back to the seamstress

This is very natural in Italian.

You could also say:

• domani torneremo dalla sarta

That is also correct, but torniamo sounds a bit more immediate and conversational.

In Italian, in a normal real condition, se is usually followed by the present indicative, not the future.

So Italian says:

• Se non riesci..., domani torniamo / torneremo...

not usually:

• Se non riuscirai... in this kind of basic structure

For an English speaker, this is important because English often thinks in terms of future meaning, but Italian commonly uses the present after se.

So the pattern here is very typical:

• Se + present
• then present with future meaning or future

Because with verbs like andare and tornare, Italian often uses da when the destination is a person or a person’s place/business.

So:

• andare dal dottore = to go to the doctor
• andare dal parrucchiere = to go to the hairdresser
• tornare dalla sarta = to go back to the seamstress

Literally, dalla sarta looks like from the seamstress, but in this kind of expression it means to the seamstress / to the seamstress’s place.

So this is a very common Italian pattern:

• da + person

Sarta means seamstress, dressmaker, or female tailor, depending on context.

It is the feminine form of:

• sarto = male tailor

So:

• dal sarto = to the tailor (male)
• dalla sarta = to the seamstress / female tailor

In this sentence, sarta suggests someone who can fix clothing, such as a broken zipper.

Because the sentence begins with the if-clause:

• Se non riesci ad aggiustare la cerniera, ...

Then the main clause comes after it:

• domani torniamo dalla sarta.

In Italian, it is normal to put a comma after an introductory conditional clause like this.

If the order were reversed, the comma might be handled differently:

• Domani torniamo dalla sarta se non riesci ad aggiustare la cerniera.

But in the original order, the comma is standard and helpful.

In this sentence, torniamo is best understood as we go back or we return.

The verb tornare often means:

• to return
• to go back
• sometimes to come back, depending on perspective

Here:

• domani torniamo dalla sarta

means that the speaker and at least one other person will go back to the seamstress again.

So go back is probably the most natural English choice in this context.