Το τρένο μπαίνει στο τούνελ.

A word-by-word breakdown:

• Το = the
  This is the neuter singular definite article.

• τρένο = train
  A neuter singular noun.

• μπαίνει = enters / goes in / is entering
  This is the 3rd person singular present form of μπαίνω (to enter / go in).

• στο = in / into / to the
  This is a contraction of σε + το.

• τούνελ = tunnel
  A neuter noun (a loanword).

So structurally, it is:

the train + enters/goes into + the tunnel

Because Greek normally uses the definite article with both nouns here:

• Το τρένο = the train
• στο τούνελ = into the tunnel / in the tunnel

The το inside στο is still the article the. So even though you only see στο, it really contains:

• σε = in / to / into
• το = the

So the sentence has two definite articles because both train and tunnel are definite.

στο is the contracted form of:

• σε + το = στο

This is extremely common in Greek.

Examples:

• στο σπίτι = in the house / to the house
• στο σχολείο = at school / to the school
• στο τούνελ = in the tunnel / into the tunnel

With a verb of motion like μπαίνω, στο often has the sense of into.

Greek often uses σε for several meanings that English separates into different prepositions, such as:

• in
• to
• into
• sometimes at

The exact meaning depends on the verb and context.

Because μπαίνω already implies movement inward, στο τούνελ is naturally understood as into the tunnel, not just in the tunnel.

So Greek does not need a separate word matching English into in this sentence.

μπαίνει is the 3rd person singular present tense of μπαίνω.

That means it is used with subjects like:

• αυτός / αυτή / αυτό = he / she / it
• or any singular noun, such as το τρένο

Examples:

• Μπαίνω = I enter / I’m entering
• Μπαίνεις = you enter / you’re entering
• Μπαίνει = he/she/it enters / is entering

Since το τρένο is singular, the verb is singular too: μπαίνει.

It can mean either, depending on context.

Greek present tense often covers both:

• simple present: enters
• present continuous: is entering

So Το τρένο μπαίνει στο τούνελ could mean:

• The train enters the tunnel
• The train is entering the tunnel

English forces you to choose more often than Greek does.

Both are neuter nouns.

You can see this from the article:

• το τρένο
• το τούνελ

This is why the contraction is στο (σε + το) and not:

• στον = σε + τον for masculine nouns
• στη(ν) = σε + την for feminine nouns

So the neuter article το is the key here.

Because τούνελ is neuter, not masculine.

Greek uses different article forms depending on gender:

• masculine: τον
• feminine: την
• neuter: το

After σε, they contract like this:

• σε τον → στον
• σε την → στη(ν)
• σε το → στο

Since τούνελ is neuter, the correct form is:

• στο τούνελ

Usually τούνελ is treated as an indeclinable loanword, which means the noun itself often stays the same, and the article shows the grammatical role.

For example, you may see:

• το τούνελ
• στο τούνελ
• από το τούνελ

The noun τούνελ often does not change, unlike many native Greek nouns.

This is common with some borrowed words.

Yes. Greek word order is more flexible than English word order.

The neutral, straightforward order is:

• Το τρένο μπαίνει στο τούνελ.

But Greek can also move parts around for emphasis, for example:

• Στο τούνελ μπαίνει το τρένο.
  Emphasis on into the tunnel

• Μπαίνει στο τούνελ το τρένο.
  Less neutral, but possible in context

Even when the order changes, the meaning often stays basically the same, though the focus changes.

μπαίνει is pronounced approximately BÉ-ni.

A few helpful points:

• At the beginning of a word, μπ is usually pronounced like b
• αι is pronounced like e
• The stress falls on μπαί-

So:

• μπαίνει ≈ BÉ-ni

This surprises many learners because the spelling μπ looks like two letters but often represents a b sound in Modern Greek.

The accent mark shows which syllable is stressed.

• τρένο → stress on τρέ-
• μπαίνει → stress on μπαί-
• τούνελ → stress on τού-

Stress matters in Greek pronunciation and is always written in normal spelling.

So the accents are not optional—they are part of the correct written form.

Yes, they could absolutely say:

• Μπαίνει στο τούνελ.

Greek often omits the subject when it is clear from context, because the verb ending already shows the person and number.

Here, μπαίνει tells you it is:

• he/she/it enters
• or a singular noun subject

But if you want to explicitly say the train, then Το τρένο μπαίνει στο τούνελ is perfectly normal.

Yes. It is simple, natural, and grammatical.

It uses very common Greek patterns:

• article + noun: το τρένο
• present tense verb: μπαίνει
• σε + article contraction: στο
• place/destination phrase: στο τούνελ

So it is a very good example sentence for learning how Greek expresses motion into a place.