Quello che amo di settembre è l’aria mite, ma a ottobre il giardino cambia colore in fretta.

Quello che means what or the thing that.

So:

• Quello che amo di settembre = What I love about September
• More literally: The thing that I love about September

This is a very common Italian structure:

• quello che penso = what I think
• quello che voglio = what I want

It is useful when you want to introduce an idea or thing in a general way.

Yes. Ciò che amo di settembre is correct and means the same thing.

The difference is mainly style:

• quello che = more common in everyday speech
• ciò che = a bit more formal or literary

In normal conversation, quello che is often the more natural choice.

Because amare qualcosa di qualcosa here means to love something about something.

Compare:

• Amo settembre = I love September
• Amo l’aria mite di settembre = I love September’s mild air
• Quello che amo di settembre = What I love about September

So di settembre does not mean you love September directly as the object of the verb. It means you love something about September.

Because aria is a singular feminine noun beginning with a vowel.

The feminine singular definite article is normally la, but before a vowel it becomes l’:

• la casa
• la strada
• l’aria
• l’acqua

So l’aria mite means the mild air.

In Italian, adjectives often come after the noun.

So:

• aria mite = mild air
• giardino grande = big garden
• tempo bello = beautiful weather

Some adjectives can go before the noun, but in many ordinary descriptive phrases, putting the adjective after the noun is the most neutral and natural choice.

So l’aria mite is the normal order.

Mite means mild, especially for weather, air, or climate.

So l’aria mite means:

• mild air
• gentle, not cold air
• pleasantly soft weather

It is a common adjective in weather-related contexts:

• clima mite = mild climate
• temperatura mite = mild temperature

Ma means but.

It connects two contrasting ideas:

• in September, the speaker loves the mild air
• in October, the garden changes color quickly

So ma shows a contrast or transition from one month to the next.

With months, Italian commonly uses a:

• a settembre
• a ottobre
• a gennaio

So a ottobre is completely normal.

A few notes:

• a ottobre = the most common everyday choice
• in ottobre = possible, but often feels a bit more formal, literary, or context-dependent
• ad ottobre = grammatically possible because ottobre starts with a vowel, but in modern usage a ottobre is much more common

So for a learner, a ottobre is the safest and most natural option.

Because cambiare colore is a common Italian expression meaning to change color.

Here colore is singular because it refers to the general state of color, not to several separate colors.

So:

• il giardino cambia colore = the garden changes color

If you said cambia colori, it would sound less natural in this context.

Italian often uses the singular in expressions like this:

• diventare rosso = to turn red
• cambiare colore = to change color

Normally, no. That would not sound natural here.

Cambiare i colori usually means to change the colors in the sense of actively replacing or altering them, as if someone were choosing different colors for something.

For example:

• Ho cambiato i colori del logo = I changed the logo’s colors

But in your sentence, the garden is naturally turning from one seasonal color pattern to another, so the idiomatic expression is:

• il giardino cambia colore

Both are possible, but they are slightly different in feel.

• in fretta = quickly, fast, in a hurry
• velocemente = quickly, rapidly

In this sentence, in fretta sounds very natural and idiomatic. It suggests that the change happens quickly over a short period.

So:

• il giardino cambia colore in fretta = the garden changes color quickly

You could also say:

• il giardino cambia colore velocemente

But in fretta often sounds a bit more everyday and expressive.

Yes. Italian adverbial expressions can often move around.

For example:

• il giardino cambia colore in fretta
• il giardino in fretta cambia colore
• in fretta il giardino cambia colore

However, the most natural and neutral order here is:

• il giardino cambia colore in fretta

That is the standard choice.

Because the sentence expresses a general observation or recurring situation, not one specific event.

Italian often uses the present tense for:

• general truths
• habits
• seasonal descriptions
• things that regularly happen

So the sentence means something like:

• what I love about September is the mild air, but in October the garden changes color quickly

It is not necessarily describing only this year; it can describe what usually happens.

Yes, very common.

This pattern is useful when you want to highlight one feature of something:

• Quello che amo di questa città è il silenzio = What I love about this city is the quiet
• Quello che mi piace del film è la musica = What I like about the movie is the music
• Quello che odio dell’estate è il caldo = What I hate about summer is the heat

It is a very practical structure to learn because it appears often in both speaking and writing.

Because Italian usually does not need subject pronouns when the verb form already makes the subject clear.

Here:

• cambia already tells you it is he/she/it changes
• the subject is clearly il giardino

So Italian naturally says:

• il giardino cambia colore

not:

• il giardino esso cambia colore

Subject pronouns such as lui, lei, or esso are used only in special cases, usually for emphasis or contrast, and esso is rare in everyday speech.