Ο λιμενικός ελέγχει την ανακοίνωση επιβίβασης.

These are definite articles, meaning the.

• ο = masculine singular nominative
• την = feminine singular accusative

They agree with the nouns they go with:

• ο λιμενικός = the subject
• την ανακοίνωση = the direct object

So the articles help show who is doing the action and what is being acted on.

Here it is being used as a noun.

Originally, λιμενικός is adjective-like in form, related to the port / harbor. But in everyday Greek, ο λιμενικός commonly means the port officer / coast guard officer / harbor police officer, depending on context.

So in this sentence, it functions as a person noun, not just as an adjective describing something else.

ελέγχει is:

• 3rd person singular
• present tense
• from the verb ελέγχω = to check / inspect / control

So it means he/she/it checks or is checking, depending on context.

Because the subject here is ο λιμενικός, the form ελέγχει means the officer checks / is checking.

Greek often leaves out subject pronouns when they are not needed.

In English, you usually need he checks.
In Greek, the verb ending already tells you the person and number, and here the subject noun is also stated:

• ο λιμενικός ελέγχει

That is enough by itself. Adding αυτός would usually sound emphatic, like he himself or he, as opposed to someone else.

Because it is the direct object of the verb.

The officer is doing the action of checking, and the thing being checked is the announcement. In Greek, direct objects normally go in the accusative case.

So:

• ο λιμενικός = subject, nominative
• την ανακοίνωση = object, accusative

Because for this noun, the nominative singular and accusative singular have the same form:

• nominative: η ανακοίνωση
• accusative: την ανακοίνωση

So the noun itself does not visibly change here, but the article does. That is very common in Greek.

The article is what clearly shows the case in this phrase.

Because Greek often uses the genitive after a noun to mean of ....

So:

• ανακοίνωση επιβίβασης literally = announcement of boarding

In natural English, that often becomes boarding announcement.

The base form is επιβίβαση.
Here it appears as the genitive singular:

• η επιβίβαση
• της επιβίβασης

Even without the article, the ending -ης shows the genitive singular.

Because Greek does not always need an article in this kind of noun + genitive phrase.

Here επιβίβασης is specifying the type of announcement:

• ανακοίνωση επιβίβασης = a boarding announcement / the boarding announcement, depending on context

If you said της επιβίβασης, that would sound more specifically like of the boarding, referring to a particular boarding event already known in the context.

So the version without the article is very natural.

No, Greek word order is more flexible than English word order.

This sentence uses a very neutral order:

• Subject + Verb + Object

So:

• Ο λιμενικός ελέγχει την ανακοίνωση επιβίβασης.

But Greek could also rearrange the words for emphasis, for example putting the object first. The articles and case endings help listeners understand the roles of the words.

Still, the order in your sentence is the most straightforward and natural one for a learner to recognize.

A rough pronunciation is:

o li-me-ni-KOS e-LENG-hi tin a-na-KI-no-si e-pi-VI-va-sis

Stress falls on:

• λιμενικός
• ελέγχει
• ανακοίνωση
• επιβίβασης

A few pronunciation notes:

• η, ι, υ, ει, οι are all pronounced like ee in Modern Greek.
• γχ in ελέγχει is a Greek consonant cluster with no exact English equivalent.
• In την, the final -ν is kept and pronounced here because the next word begins with a vowel.

If you wanted to look the words up, the usual dictionary forms are:

• λιμενικός = port/coast guard officer
• ελέγχω = to check
• ανακοίνωση = announcement
• επιβίβαση = boarding

That is especially useful because ελέγχει and επιβίβασης are not the basic dictionary forms.

Yes.

The Greek present tense often covers both ideas:

• checks = habitual or general present
• is checking = action happening now

So ελέγχει can mean either one, and context tells you which is intended. Greek does not always make the same present-time distinction that English does.