In autunno le foglie diventano gialle.

In autunno is the most natural way to say in autumn / in the fall when you mean the season in general.

Italian normally uses in + season with no article for general statements:

• in primavera
• in estate
• in autunno
• in inverno

You may sometimes hear nell'autunno, but that usually sounds more specific, literary, or tied to a particular autumn, such as nell'autunno del 2023.

So in a general sentence like this one, in autunno is exactly what you expect.

Italian uses the definite article much more often than English.

Here, le foglie means leaves in a general sense. Italian often uses the article when talking about things as a class or category:

• I cani sono fedeli = Dogs are loyal
• Le rose sono belle = Roses are beautiful
• Le foglie diventano gialle = Leaves turn yellow

So even though English often drops the in general statements, Italian usually keeps the article.

The base verb is diventare, meaning to become / to turn.

Diventano is the third person plural present tense form, used because the subject is le foglie = the leaves, which is plural.

Present tense of diventare:

• io divento
• tu diventi
• lui/lei diventa
• noi diventiamo
• voi diventate
• loro diventano

So:

• la foglia diventa gialla = the leaf turns yellow
• le foglie diventano gialle = the leaves turn yellow

Because adjectives in Italian must agree with the noun they describe.

Here the noun is le foglie:

• foglia = singular feminine
• foglie = plural feminine

So the adjective giallo has to match:

• giallo = masculine singular
• gialla = feminine singular
• gialli = masculine plural
• gialle = feminine plural

That is why the sentence uses gialle.

Italian, like English, often uses the present tense for general truths, habits, and regularly recurring events.

So In autunno le foglie diventano gialle means something like:

• In autumn, leaves turn yellow
• Every autumn, the leaves turn yellow

It is not necessarily talking about what is happening right now. It is stating a normal fact.

Because diventano expresses a change.

• sono gialle = they are yellow
• diventano gialle = they become / turn yellow

Leaves are not yellow all the time; they change color in autumn. So diventano is the better verb here.

A similar contrast:

• Le banane sono gialle = Bananas are yellow
• Le foglie diventano gialle = Leaves turn yellow

Yes. That version is also correct.

Both are natural:

• In autunno le foglie diventano gialle
• Le foglie diventano gialle in autunno

The difference is mainly emphasis:

• In autunno... puts the time first, like In autumn...
• Le foglie... starts with the subject, like The leaves...

Italian word order is often more flexible than English, especially with time expressions.

Yes, normally autunno is lowercase, just like the names of seasons, months, and days in Italian.

So you write:

• in autunno
• a settembre
• di lunedì

not with capitals, unless the word begins the sentence or is part of a title.

Foglie can be tricky for English speakers.

It is pronounced roughly like FOH-lyeh, with gli making a special Italian sound. It is not pronounced like English fog-lee.

A helpful breakdown:

• fo-
• -glie with the gli sound, similar to the lli in some pronunciations of million, but more distinctly Italian

Also:

• foglia = singular
• foglie = plural

That gli spelling appears in many Italian words, so it is worth practicing.

Italian usually does not use subject pronouns unless they are needed for emphasis or contrast.

The verb ending already shows the person and number:

• diventano tells you the subject is they

But in this sentence the noun le foglie is already stated, so adding loro would be unnecessary.

Italian prefers:

• Le foglie diventano gialle

not:

• Loro le foglie diventano gialle ❌

and usually not even:

• Le foglie loro diventano gialle ❌

You might hear similar ideas in some contexts, but diventano gialle is the most standard and straightforward choice here.

For a learner, the safest natural option is:

• Le foglie diventano gialle

because it clearly means the leaves turn/become yellow.

Farsi can sometimes express becoming, but it is more idiomatic and context-dependent, so it is better to learn diventare first for this kind of sentence.