L’ananas in frigo è per la merenda.

Because ananas begins with a vowel, and the singular definite article becomes l’ before a vowel. Also, ananas is normally treated as masculine in Italian, so the standard form is l’ananas.

You would also say:

• un ananas
• l’ananas

Not il ananas, because il does not stay unchanged before a vowel.

It is singular here.

You can tell from:

• the article l’
• the verb è, which means is and is singular

A useful thing to know is that ananas is usually invariable, so the noun often looks the same in the singular and plural:

• singular: l’ananas
• plural: gli ananas

So the article and verb help you understand the number.

In frigo is a very common everyday Italian expression meaning in the fridge. Italian often uses this shorter pattern without the article in fixed, familiar expressions like this.

You can also hear:

• nel frigo
• nel frigorifero
• in frigorifero

But in frigo is very natural and common in spoken Italian.

Yes. Frigo is a shortened form of frigorifero.

That said, it is extremely common and completely normal in everyday speech. It is not strange or incorrect. It is just less formal than frigorifero.

So:

• frigo = everyday, very common
• frigorifero = fuller, more formal or careful

Merenda means a light meal or snack, often eaten in the afternoon. It is especially associated with children, but adults use the word too.

It is close to snack, but it can also suggest a regular snack time, not just any small thing to eat at any moment. So it often has a slightly more specific feel than the English word snack.

Here per means for in the sense of intended for, meant for, or saved for.

So è per la merenda means the pineapple is intended for the snack time.

The article la makes it sound like a specific snack occasion, such as the expected afternoon snack. You may also hear per merenda, which is a bit more general.

In this sentence, it most naturally describes which pineapple we are talking about: the pineapple in the fridge.

So the structure is basically:

• L’ananas in frigo = the pineapple in the fridge
• è per la merenda = is for the snack

If you wanted to make the location the main statement, you would more naturally say L’ananas è in frigo.

Because è is the verb is from essere.

Without the accent, e means and.

So:

• è = is
• e = and

The accent is important because it distinguishes two different words.

Yes. A few natural alternatives are:

• L’ananas è in frigo per la merenda.
• L’ananas nel frigo è per la merenda.

These are slightly different in focus, but very close in meaning.

The original sentence is compact and natural. It treats in frigo as part of the noun phrase l’ananas in frigo, then says that this pineapple is for the snack.