A dicembre fa buio presto e io torno presto a casa.

With months, Italian normally uses a:

• a dicembre = in December
• a gennaio = in January

Usually there is no article before the month in this general meaning.

You may see other forms in more specific contexts:

• nel dicembre 2023 = in December 2023
• in dicembre is much less common and usually not the standard choice in everyday speech

So a dicembre is the natural way to say in December here.

Italian often does not use a dummy subject like English it in expressions about weather, time, or general conditions.

So English says:

• it gets dark early

But Italian simply says:

• fa buio presto

The verb is just used impersonally. There is no need to add a subject pronoun.

Fa buio is a very common Italian expression meaning it gets dark or it becomes dark.

Literally, it looks like makes dark/darkness, because fare usually means to do or to make, but here you should learn it as a fixed expression:

• fa buio = it gets dark
• quando fa buio = when it gets dark

A useful comparison:

• è buio = it is dark
• fa buio = it gets dark / it becomes dark

So in your sentence, the idea is about darkness arriving early, not just the state of being dark.

In fa buio, buio behaves like a noun in an idiomatic expression: essentially darkness.

That said, learners do not usually need to analyze it too deeply. The most useful thing is to remember the whole phrase:

• fare buio = to get dark

You will also see buio used in other ways:

• È buio. = It’s dark.
• Ho paura del buio. = I’m afraid of the dark.

So it can appear in different grammatical roles depending on the sentence.

Presto can mean both early and soon, depending on context.

In this sentence, both uses are best understood as early:

• fa buio presto = it gets dark early
• torno presto a casa = I come home early

So yes, repeating presto is completely normal. It is modifying two different parts of the sentence.

Examples of the other meaning:

• Arrivo presto. = I’ll arrive soon.
• Mi alzo presto. = I get up early.

Context tells you which meaning is intended.

That’s true: Italian often omits subject pronouns when they are clear from the verb ending.

So this sentence could also be:

• A dicembre fa buio presto e torno presto a casa.

That would still be correct.

Using io adds a bit more emphasis or contrast, as if the speaker is saying:

• and I come home early
• and as for me, I come home early

So io is not required, but it is perfectly natural.

Because a casa is the normal expression for home.

• tornare a casa = to return home
• essere a casa = to be at home
• andare a casa = to go home

When Italian means home in this general personal sense, it usually uses a casa without an article.

Alla casa would mean something more like to the house/building, which is different and much less natural here.

So:

• torno a casa = I return home
• torno alla casa = I return to the house

Because the Italian present tense is often used for:

• habits
• routines
• general truths
• things that usually happen

So torno presto a casa can mean:

• I come home early
• I usually come home early
• I return home early

In this sentence, the meaning is habitual: in December, it gets dark early, and the speaker typically goes home early.

This is very normal Italian usage.

Yes, Italian word order is somewhat flexible, though the original version sounds very natural:

• A dicembre fa buio presto e io torno presto a casa.

You could move some parts around for emphasis, for example:

• Io torno presto a casa a dicembre.
• A dicembre io torno presto a casa.

But the original order is smooth and standard because:

• A dicembre sets the time frame first
• fa buio presto gives the general situation
• e io torno presto a casa adds the speaker’s response or habit

So other orders are possible, but the given one is a very good, natural model.