Il postino lascia una lettera nella cassetta delle lettere.

Lascia is the third-person singular present of lasciare (to leave). It matches il postino (he/the mailman).

So:

• io lascio = I leave
• tu lasci = you leave
• lui/lei lascia = he/she leaves

In this sentence, Il postino lascia... = The mailman leaves...

Una is the feminine singular indefinite article, meaning a or an.
Lettera is feminine singular, so una lettera = a letter.

If you said la lettera, it would mean the letter, referring to a specific letter already known from the context.

Because nouns in Italian have grammatical gender. Lettera is a feminine noun, so it takes feminine articles:

• una lettera
• la lettera

This does not mean a letter is biologically female; it is just a grammatical category.

Nella means in the. It is a combination of:

• in = in
• la = the

Together:

• in + la = nella

Italian often combines prepositions with definite articles:

• nel = in + il
• nello = in + lo
• nella = in + la
• nei / negli / nelle = plural forms

So nella cassetta = in the box.

Cassetta delle lettere is a fixed expression meaning mailbox or letter box.

Literally, it is:

• cassetta = small box
• delle lettere = of the letters

So the structure is roughly box of letters, which Italian uses for mailbox/letter box.

Here delle means of the. It is made from:

• di = of
• le = the (feminine plural)

So:

• delle lettere = of the letters

In cassetta delle lettere, this is part of the standard Italian expression for mailbox.

Good question. The plural lettere belongs to the expression cassetta delle lettere, which names the object itself: mailbox.

So:

• una lettera = one letter
• cassetta delle lettere = mailbox, literally box of letters

The mailbox is a place for letters in general, so the plural is natural in Italian.

They are related in meaning, but they do different jobs in the sentence.

• una lettera is the direct object: the thing being left.
• delle lettere is part of the noun phrase cassetta delle lettere, the place where the letter is being left.

So the sentence is saying that the mailman leaves one letter in the mailbox.

This is the normal, neutral Italian word order: subject + verb + object + place.

• Il postino = subject
• lascia = verb
• una lettera = object
• nella cassetta delle lettere = place

Italian word order can change for emphasis, but this version is the most straightforward and natural for a basic statement.

Yes. Lasciare can mean several things depending on context, such as:

• to leave
• to put down
• to let
• to abandon

In this sentence, because of una lettera nella cassetta delle lettere, the meaning is clearly leaves/posts a letter in the mailbox.

Postino is masculine singular. A feminine form is postina.

So:

• il postino = the male mailman/postman
• la postina = the female mail carrier/postwoman

The sentence would become: La postina lascia una lettera nella cassetta delle lettere.

Usually cassetta delle lettere means mailbox or letterbox.
A mail slot is usually something more specific, like a slot in a door, and Italian would often use a different expression depending on the exact object.

So for most learners, the safest translation here is mailbox or letterbox.

It is a very common pattern, but not a perfect rule:

• nouns ending in -o are often masculine: postino
• nouns ending in -a are often feminine: lettera

This pattern helps a lot, but there are exceptions in Italian, so it is best to learn each noun together with its article:

• il postino
• la lettera

A learner might reduce it to these core pieces:

• il postino = the mailman
• lascia = leaves
• una lettera = a letter
• nella cassetta delle lettere = in the mailbox

That shows the sentence structure very clearly and helps you see how each part works.