Allarghiamo la scarpa finché calza bene.

It can mean either, depending on context.

• Noi allarghiamo = we widen/stretch
• Allarghiamo! = let’s widen/stretch!

In this sentence, the most natural reading is the exhortative one: let’s stretch the shoe until it fits well.

Italian often uses the noi form this way, where English uses let’s.

Because the verb is allargare.

With verbs ending in -gare, Italian adds h before e or i to keep the hard g sound.

Compare:

• allargo
• allarghi
• allarghiamo

Without the h, gi would sound different. So allarghiamo keeps the pronunciation consistent.

Italian usually uses an article with nouns more often than English does.

So la scarpa is normal Italian for the shoe. It usually refers to a specific shoe already known from the situation.

Using no article here would sound incomplete or unnatural in standard Italian.

Not necessarily. La scarpa means the shoe, so the sentence is talking about one shoe.

That could be because:

• only one shoe needs stretching
• the speaker is referring to each shoe individually
• the context is focused on a single shoe

If both shoes were meant, you would normally say:

• Allarghiamo le scarpe finché calzano bene.

The understood subject is la scarpa.

So the second part means:

• finché la scarpa calza bene = until the shoe fits well

Italian often leaves out repeated subjects when they are already clear from context. So instead of repeating la scarpa, it is simply understood.

Yes. Calzare is commonly used for clothing items—especially shoes, but also things like gloves, hats, or clothes—to mean to fit.

So:

• La scarpa calza bene = The shoe fits well
• Queste scarpe calzano strette = These shoes fit tight

It is more specific than a very general verb like andare bene. With shoes, calzare bene sounds especially natural.

Here finché means until.

So:

• Allarghiamo la scarpa finché calza bene
  = Let’s stretch the shoe until it fits well

In other contexts, finché can also mean as long as, depending on the sentence.

Because the stress falls on the final syllable: fin-CHÉ.

In Italian, words stressed on the last vowel normally take a written accent. So finché must be written with é.

This helps both spelling and pronunciation.

Because this is a straightforward, real result the speaker is aiming for: stretch the shoe until it fits well.

Italian commonly uses the indicative after finché in sentences like this, especially when the clause expresses a concrete state or result.

So calza bene is perfectly natural here.

Yes, finché non calza bene is also possible in Italian, and in this kind of sentence it can still mean until it fits well, not until it doesn’t fit well.

That non can be what learners sometimes call an expletive non: it appears after words like finché without making the meaning negative.

So:

• finché calza bene
• finché non calza bene

can both mean until it fits well, depending on context.

In your sentence, leaving out non is completely normal and clear.

Only if the structure of the sentence changes.

Allarghiamo is a finite verb form, so it can stand as the main verb of the sentence:

• Allarghiamo la scarpa finché calza bene.

Allargare is the infinitive, so it would need a different structure, for example:

• Dobbiamo allargare la scarpa finché calza bene.
• Bisogna allargare la scarpa finché calza bene.

So you cannot simply replace allarghiamo with allargare without adjusting the rest of the sentence.