Chissà quando arriverà il treno.

In this sentence, chissà means something like who knows or I wonder.

So Chissà quando arriverà il treno is literally close to Who knows when the train will arrive.

A useful thing to remember:

• chissà often introduces uncertainty, doubt, or curiosity
• it is very common in spoken Italian

Examples:

• Chissà dov’è Maria. = Who knows where Maria is.
• Chissà se verranno. = I wonder if they’ll come.

It is normally written as one word: chissà.

Historically, it comes from chi sa (who knows), but in modern Italian the common form in this meaning is the fixed expression chissà.

So for learners, it is best to treat chissà as a set phrase meaning:

• who knows
• I wonder

Arriverà is the simple future of arrivare:

• arriverà = he/she/it will arrive

Italian often uses the future here because the arrival is still in the future from the speaker’s point of view.

Breakdown:

• quando = when
• arriverà = will arrive
• il treno = the train

So:

• quando arriverà il treno = when the train will arrive

In natural English, we usually say when the train arrives, not when the train will arrive, but Italian commonly keeps the future tense in this type of sentence.

Yes, you may hear arriva instead of arriverà, especially in everyday speech.

Difference:

• Chissà quando arriverà il treno sounds a bit more explicitly future-oriented
• Chissà quando arriva il treno can sound more conversational and immediate

Both are understandable and natural in many contexts, but arriverà is a very standard choice when talking about a future event.

Both word orders are possible in Italian.

• quando arriverà il treno
• quando il treno arriverà

The version with the verb before the subject is very natural after question words like quando, especially in indirect questions.

So quando arriverà il treno sounds very normal and idiomatic.

Italian is often more flexible than English with word order, especially when the subject is a noun rather than a pronoun.

Because chissà can be followed directly by:

• quando
• dove
• come
• chi
• se, etc.

So:

• Chissà quando arriverà il treno
• Chissà dove sono
• Chissà se viene

You do not need che in this structure.

English learners sometimes expect something like who knows that/when..., but Italian simply says chissà + question word / se.

It is an indirect question.

A direct question would be:

• Quando arriverà il treno? = When will the train arrive?

An indirect question is embedded inside another expression:

• Chissà quando arriverà il treno. = Who knows when the train will arrive.

So the sentence contains:

1. chissà = expression of uncertainty
2. quando arriverà il treno = indirect question

Italian usually uses the definite article much more often than English.

Here il treno means the train:

• il = masculine singular definite article
• treno = train

Even when English might sometimes be more flexible, Italian often prefers the article with a specific noun:

• Aspetto il treno. = I’m waiting for the train.
• Il treno è in ritardo. = The train is late.

It is third person singular.

From arrivare:

• io arriverò = I will arrive
• tu arriverai = you will arrive
• lui/lei arriverà = he/she will arrive
• esso/it arriverà = it will arrive

Since il treno is singular, the verb is singular too:

• il treno arriverà

The accent mark on arriverà shows that the stress falls on the last syllable:

• ar-ri-ve-rà

This is normal for many future-tense forms in Italian, especially:

• sarà
• andrà
• verrà
• arriverà

The written accent is important because it is part of the correct spelling.

A simple pronunciation guide is:

• chissà ≈ kee-SSAH

Notes:

• chi sounds like kee
• the stress is on the last syllable: -sà
• the doubled ss is pronounced more strongly than a single s

So the rhythm is roughly:

• chi-SSÀ

It is neutral and very common. You can use it in:

• everyday conversation
• writing
• fairly neutral contexts

It is not slang, and it is not especially formal either.

So it is a very useful expression to learn.

Yes. Very often.

If someone asks a question and you do not know the answer, you can simply say:

• Chissà! = Who knows!

It can also begin longer sentences:

• Chissà dov’è andato.
• Chissà se ci chiameranno.
• Chissà quando passa.

Yes, absolutely. Mah is a very common discourse word meaning something like:

• well...
• hmm...
• who knows...

So:

• Mah, chissà quando arriverà il treno.

This sounds very natural in conversation if the speaker is annoyed, doubtful, or uncertain.

Not exactly. It is more of a comment than a direct request for information.

Compare:

Direct question:

• Quando arriverà il treno? = someone is asking for the answer

Comment/exclamation:

• Chissà quando arriverà il treno. = the speaker is wondering, but not necessarily asking someone to answer

So the sentence expresses uncertainty or speculation rather than a straightforward question.

Yes. Depending on tone and context, it can sound:

• neutral: simple uncertainty
• curious: genuine wondering
• frustrated: especially if the train is late

For example, if someone has been waiting a long time, Chissà quando arriverà il treno may sound like:

• Who knows when this train is ever going to arrive

So tone of voice matters a lot.