Ο καταπέλτης ανοίγει στο κατάστρωμα.

Ο is the masculine singular definite article, so it means the.

So:

• ο καταπέλτης = the catapult

Greek uses the definite article more often than English does. In a normal statement about a specific thing, Greek usually includes it.

Καταπέλτης is in the nominative singular, because it is the subject of the sentence.

It belongs to a common masculine noun pattern:

• nominative singular: ο καταπέλτης
• genitive singular: του καταπέλτη
• accusative singular: τον καταπέλτη

So the ending -ης here is not a special ending for this sentence in particular; it is just the normal dictionary form for this type of masculine noun.

Ανοίγει is the 3rd person singular present tense form of ανοίγω.

That means it matches a singular subject like ο καταπέλτης:

• ανοίγω = I open
• ανοίγεις = you open
• ανοίγει = he/she/it opens

In context, Greek present tense can correspond to several English possibilities, depending on the situation:

• opens
• is opening
• sometimes does open

Here, the subject is a thing, so in English it would usually be it opens / the catapult opens.

Στο is the normal contraction of:

• σε + το = στο

So:

• στο κατάστρωμα = on the deck / to the deck / at the deck, depending on context

This contraction is extremely common in Greek, just like:

• στον = σε + τον
• στη = σε + τη(ν)
• στην = σε + την

You should think of στο as a very standard form, not as slang or an optional shortcut.

Because σε normally takes the accusative case in Modern Greek.

The noun κατάστρωμα is a neuter noun, and in the singular its nominative and accusative forms are the same:

• nominative: το κατάστρωμα
• accusative: το κατάστρωμα

So after στο (σε + το), you get:

• στο κατάστρωμα

Even though the form looks unchanged, it is functioning as accusative after the preposition.

It can mean different things in English depending on context.

The Greek preposition σε is very broad. It can correspond to:

• in
• on
• at
• to

So στο κατάστρωμα could be translated as on the deck, at the deck, or even to the deck, depending on what the full context is supposed to mean.

English often forces you to choose one precise preposition, but Greek σε is more flexible.

No, Greek word order is more flexible than English.

The neutral order here is:

• Ο καταπέλτης ανοίγει στο κατάστρωμα.

But Greek could also rearrange parts of the sentence for emphasis or style, for example:

• Στο κατάστρωμα ανοίγει ο καταπέλτης.

That does not automatically change the basic meaning. The endings, articles, and overall structure help show what each word is doing.

English relies much more heavily on word order to show subject and object, but Greek relies more on morphology.

Normally, no—not in an ordinary sentence like this.

Greek usually wants the article with a specific countable noun used as the subject:

• Ο καταπέλτης = normal
• Καταπέλτης by itself would sound unusual here

Leaving the article out might happen in special contexts such as:

• headlines
• labels
• dictionary entries
• very compressed or poetic language

So for a regular sentence, ο καταπέλτης is the form you would expect.

Yes.

Just like English open, Greek ανοίγω can be used:

• transitively: someone opens something
• intransitively: something opens

Examples:

• Ανοίγω την πόρτα. = I open the door.
• Η πόρτα ανοίγει. = The door opens / is opening.

In your sentence, ανοίγει is being used intransitively:

• Ο καταπέλτης ανοίγει = The catapult opens

So there is no direct object in this sentence.

A simple pronunciation guide would be:

• Ο καταπέλτης ανοίγει στο κατάστρωμα
• o ka-ta-PEL-tis a-NI-yi sto ka-TA-stro-ma

A few useful notes:

• The stress marks show which syllable gets emphasis:
  • καταπέλτης → -πέλ-
  • ανοίγει → -νοί-
  • κατάστρωμα → -κά-
• γ before ι in ανοίγει sounds like a soft sound, roughly like y in a simplified learner-friendly pronunciation.

So ανοίγει is often approximated for beginners as a-NI-yi.