Se la federa del cuscino è vecchia, ne compro un’altra.

Federa means pillowcase.

So:

• cuscino = pillow
• federa del cuscino = the pillow’s pillowcase / the pillowcase of the pillow

In natural English, we just say pillowcase, but Italian often expresses this as federa del cuscino.

Del is a contraction of di + il:

• di = of
• il = the
• di + il = del

So:

• la federa del cuscino = the pillowcase of the pillow

Italian often uses this di structure where English uses a noun as an adjective, like:

• la porta della casa = the house door / the door of the house
• il colore del mare = the color of the sea

Italian uses articles more often than English.

Here:

• la federa = the pillowcase
• del cuscino = of the pillow

In English, we might say the pillowcase is old, without repeating the idea of the inside the phrase. But in Italian, federa and cuscino are both nouns, so each part keeps its own article structure.

Because federa is a feminine singular noun.

Adjectives in Italian usually agree with the noun they describe:

• masculine singular: vecchio
• feminine singular: vecchia
• masculine plural: vecchi
• feminine plural: vecchie

Since federa is feminine singular, you get:

• la federa è vecchia

Here, ne means something like of it / of them / one of that kind.

It refers back to federa. So:

• ne compro un’altra = I buy another one
• more literally: I buy another one of it/of them

Italian often uses ne when replacing a noun after quantities, numbers, or words like uno, due, alcuni, un altro, etc.

Examples:

• Hai mele? Ne voglio due. = Do you have apples? I want two.
• La federa è vecchia, ne compro un’altra. = The pillowcase is old, I’ll buy another one.

Yes, that is possible, and it sounds natural too.

• ne compro un’altra is a bit more explicit in showing that un’altra means another one of that thing
• compro un’altra also works because the meaning is clear from context

So both are acceptable:

• Se la federa del cuscino è vecchia, ne compro un’altra.
• Se la federa del cuscino è vecchia, compro un’altra.

Using ne is very idiomatic Italian.

Because it refers to federa, which is feminine.

Altro agrees with the noun:

• un altro cuscino = another pillow
  masculine
• un’altra federa = another pillowcase
  feminine

Even though the noun federa is not repeated, un’altra still has to match it.

Because una becomes un’ before a feminine noun or adjective starting with a vowel.

So:

• una federa
• un’altra federa

This is elision, which helps pronunciation.

Compare:

• un altro cuscino = masculine, no apostrophe
• un’altra federa = feminine, apostrophe

This is an important distinction:

• un can be masculine singular
• un’ clearly shows feminine singular before a vowel

In Italian, the present tense is often used for a future or likely result, especially in everyday speech.

So:

• Se la federa del cuscino è vecchia, ne compro un’altra.

can mean:

• If the pillowcase is old, I buy another one
• more naturally in English: If the pillowcase is old, I’ll buy another one

Italian does this very often. The present tense can express a future action when the context makes it clear.

You could also say:

• Se la federa del cuscino è vecchia, ne comprerò un’altra.

That is also correct, but the original sentence sounds very natural.

Because this is a normal, real condition, so Italian uses the indicative after se.

• Se la federa del cuscino è vecchia... = If the pillowcase is old...

This is a straightforward condition, not doubt, emotion, or opinion, so no subjunctive is needed.

A very common pattern is:

• Se + present indicative, present/future
• Se piove, resto a casa.
• Se ho tempo, ti chiamo.

It is a real or possible condition.

The structure is:

• Se
  • present indicative
• main clause in present indicative

So it means something like:

• If the pillowcase is old, I buy / I’ll buy another one

It suggests a normal, realistic possibility.

For a more hypothetical or unlikely situation, Italian would use a different pattern, for example:

• Se la federa fosse vecchia, ne comprerei un’altra.
• If the pillowcase were old, I would buy another one.

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

• compro = I buy
• compri = you buy
• compra = he/she buys

So io is not necessary.

You could say:

• ...io ne compro un’altra

but that would usually add emphasis or contrast, for example:

• Tu la tieni, io ne compro un’altra.
  You keep it, I’ll buy another one.

It is:

• Se
  • condition
• result clause

So:

• Se la federa del cuscino è vecchia = condition
• ne compro un’altra = result

Literally:

• If the pillowcase of the pillow is old, of-it I buy another

Natural English:

• If the pillowcase is old, I’ll buy another one.

Yes. Italian can put the main clause first:

• Ne compro un’altra se la federa del cuscino è vecchia.

This still means:

• I’ll buy another one if the pillowcase is old.

Both orders are correct. Starting with Se... is very common when you want to present the condition first.

Yes, cuscino is masculine singular:

• il cuscino = the pillow
• i cuscini = the pillows

That is why di + il becomes del:

• la federa del cuscino

If the noun after di were feminine, the contraction would change, for example:

• di + la = della

A plural version could be:

• Se le federe dei cuscini sono vecchie, ne compro delle altre.

Breakdown:

• le federe = the pillowcases
• dei cuscini = of the pillows
• sono vecchie = are old
• ne compro delle altre = I buy some others / some more

This shows the same agreement patterns in plural form.

The key points are:

• federa is feminine, so adjectives and related words must match: vecchia, un’altra
• del = di + il
• ne replaces the noun idea and is common with expressions like un’altra
• Italian often uses the present tense where English uses will
• subject pronouns like io are often omitted
• after se in a real condition, Italian uses the indicative: se è

It is a short sentence, but it shows several very common Italian patterns.