Ο υπνόσακος είναι μέσα στη σκηνή στον κατασκηνωτικό χώρο.

Ο is the masculine singular definite article, meaning the.

So:

• ο υπνόσακος = the sleeping bag

Greek articles change for gender, number, and case. Here υπνόσακος is a masculine singular noun, so the nominative singular article is ο.

Υπνόσακος means sleeping bag.

It is made up of:

• ύπνος = sleep
• σάκος = bag / sack

So it literally means something like sleep-bag.

Είναι means is.

It is the 3rd person singular present tense of είμαι = to be.

So:

• Ο υπνόσακος είναι... = The sleeping bag is...

Unlike Russian or Arabic, modern Greek normally does use the verb to be in sentences like this.

Μέσα means inside, so μέσα στη σκηνή is more specific than just στη σκηνή.

Compare:

• στη σκηνή = in the tent
• μέσα στη σκηνή = inside the tent

Greek often uses μέσα σε or the contracted form μέσα στη / στον / στο to make the idea of physical interior very clear.

Because after a preposition like σε or μέσα σε, Greek uses the accusative case, not the nominative.

So:

• nominative: η σκηνή = the tent (subject form)
• accusative: τη(ν) σκηνή = the tent (object / after preposition)

With σε, this usually contracts:

• σε τη σκηνή → στη σκηνή

So μέσα στη σκηνή is the normal form.

Because χώρος is a masculine noun.

With masculine singular accusative nouns, the article is τον, and with σε it contracts to στον:

• σε τον χώρο → στον χώρο

By contrast, στο is used with neuter nouns:

• στο σπίτι = in the house
• στο δωμάτιο = in the room

So:

• στον κατασκηνωτικό χώρο = correct
• στο κατασκηνωτικό χώρο = not correct here

This is a normal masculine noun pattern.

Many masculine nouns ending in -ος change to -ο in the singular accusative:

• nominative: ο χώρος
• accusative: τον χώρο

Since the phrase is after σε, it needs the accusative:

• στον κατασκηνωτικό χώρο

Because adjectives must agree with the noun they describe in gender, number, and case.

The basic form is:

• κατασκηνωτικός = masculine nominative singular

But here it describes χώρο, which is masculine accusative singular, so the adjective must also be masculine accusative singular:

• κατασκηνωτικό χώρο

This is exactly like the noun’s case change.

It literally means camping area or campground / campsite area.

• κατασκηνωτικός = related to camping
• χώρος = area / space / place

A natural English translation might be:

• campsite
• campground
• camping area

The Greek phrase sounds a bit more descriptive and formal than the everyday loanword κάμπινγκ, which is also very common in Greek.

Because they give two layers of location:

• μέσα στη σκηνή = the sleeping bag is inside the tent
• στον κατασκηνωτικό χώρο = the tent/location is at the campsite

So the whole sentence places the sleeping bag very specifically: it is inside the tent, and that tent is in the camping area.

This kind of stacked location is completely normal in Greek.

Yes, if the subject is already understood from context.

Greek often drops things that are obvious. So if everyone already knows you are talking about the sleeping bag, you could say:

• Είναι μέσα στη σκηνή. = It’s inside the tent.

But if you want to state clearly what is being talked about, keeping Ο υπνόσακος is natural.

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

The neutral order here is:

• Ο υπνόσακος είναι μέσα στη σκηνή στον κατασκηνωτικό χώρο.

But Greek can move things around for emphasis, for example:

• Μέσα στη σκηνή είναι ο υπνόσακος.
• Στον κατασκηνωτικό χώρο, ο υπνόσακος είναι μέσα στη σκηνή.

The original sentence is a straightforward, natural way to say it.

Yes, you may hear both.

The full form is based on στην:

• σε την σκηνή → στην σκηνή

But in everyday Greek, the final -ν is often dropped before certain consonants, giving:

• στη σκηνή

So both of these can occur:

• μέσα στη σκηνή
• μέσα στην σκηνή

In everyday speech, μέσα στη σκηνή is very common.

A rough pronunciation is:

O ipnó-sakos íne mésa sti skiní ston kataskinotikó chóro.

A few helpful points:

• υ here sounds like i
• η, ι, υ, ει, οι all usually sound like ee
• σκηνή sounds roughly like skee-NEE
• είναι in normal speech often sounds close to EE-ne

The stressed syllables are marked by the accent marks:

• υπνόσακος
• είναι
• μέσα
• σκηνή
• κατασκηνωτικό
• χώρο