Το τούνελ είναι κάτω από το βουνό.

Because each noun has its own definite article:

• το τούνελ = the tunnel
• το βουνό = the mountain

Greek uses the definite article very regularly, so if both nouns are definite, both usually need an article.

Both τούνελ and βουνό are neuter nouns, so they take the neuter singular article το.

• το τούνελ
• το βουνό

This is why the article is the same before both nouns.

είναι is the present-tense form of the verb to be. In this sentence it means is.

So:

• Το τούνελ είναι... = The tunnel is...

A useful thing to know is that είναι is also used for he/she/it is and they are, depending on context.

In Modern Greek, κάτω από is the normal way to say under / below / beneath in a physical location sense.

• κάτω = down / below
• από = from, but in combinations like this it helps form the full prepositional meaning under

So you should learn κάτω από as a fixed expression meaning under.

Usually it refers to position, especially physical location:

• κάτω από το τραπέζι = under the table
• κάτω από το βουνό = under the mountain

It can also be used more figuratively in some contexts, but for a learner, the safest first meaning is under/below.

Good question. In Modern Greek, after most prepositions, the noun phrase usually appears in the accusative case.

Here, από takes the accusative. But with a neuter singular noun like βουνό, the nominative and accusative forms are the same:

• nominative: το βουνό
• accusative: το βουνό

So you do not see any visible change here.

If it were masculine or feminine, the article would show the case more clearly:

• κάτω από τον δρόμο = under the road
• κάτω από τη γέφυρα = under the bridge

No, τούνελ is a loanword, ultimately from a foreign source, like English tunnel.

A useful point for learners is that τούνελ is commonly treated as an indeclinable noun in everyday Modern Greek, which means its form usually stays the same:

• το τούνελ
• του τούνελ
• στο τούνελ
• τα τούνελ

So unlike many Greek nouns, it does not change much in form.

A simple learner-friendly pronunciation guide would be:

to TOO-nel EE-ne KAH-to a-PO to voo-NO

The stressed syllables are shown in capitals. The written accent marks in Greek tell you where the stress goes:

• τούνελ
• είναι
• κάτω
• από
• βουνό

In Modern Greek, accent marks show which syllable is stressed.

For example:

• τούνελ → stress on τού-
• είναι → stress on εί-
• κάτω → stress on κά-
• από → stress on -πό
• βουνό → stress on -νό

These marks are very important in Greek spelling, and they help both pronunciation and reading.

The given order is the most neutral and natural:

• Το τούνελ είναι κάτω από το βουνό.

Greek word order is more flexible than English, so changes are possible for emphasis, but not all versions sound equally neutral.

For example:

• Κάτω από το βουνό είναι το τούνελ.
  This is possible, but it sounds more marked, like It is under the mountain that the tunnel is or as if you are emphasizing location.

For everyday use, the original order is the best one to learn first.

Not in normal standard Greek if you mean the tunnel and the mountain.

Greek normally uses the article with definite nouns, so:

• Το τούνελ = the tunnel
• το βουνό = the mountain

Leaving the articles out would sound unnatural here.

Normally, no. In a sentence like this, Greek keeps the verb είναι.

So:

• Το τούνελ είναι κάτω από το βουνό. = correct

Leaving out είναι would not sound like a normal complete sentence in standard Modern Greek.

For ordinary modern usage, κάτω από το βουνό is the most natural choice.

You may encounter other expressions in very formal, literary, or specialized contexts, but for everyday Modern Greek, κάτω από is what learners should use.

So if you want to say under the mountain, κάτω από το βουνό is the right phrase to remember.