Ο επικεφαλής είναι παρών.

ο is the masculine singular definite article, meaning the.

So ο επικεφαλής means the head / the person in charge / the chief.

Greek uses the definite article very often with roles, titles, and people, sometimes more often than English does.

επικεφαλής means head, chief, or the person in charge.

In this sentence, it works like a noun: the head / the leader.

A useful thing to know is that επικεφαλής can also be used adjectivally in other contexts, but here it clearly refers to a person.

Because επικεφαλής is an indeclinable word in Modern Greek in this use. That means its form usually stays the same.

So you can have:

• ο επικεφαλής = the male head / chief
• η επικεφαλής = the female head / chief
• οι επικεφαλής = the heads / chiefs

The article, and sometimes the rest of the sentence, tells you the gender and number.

είναι is the 3rd person singular present of είμαι, which means to be.

Here it means is.

So the structure is:

• Ο επικεφαλής = the head
• είναι = is
• παρών = present

Because παρών must agree with the subject.

Here, the subject is ο επικεφαλής, which is masculine singular, so the adjective is also masculine singular:

• masculine: παρών
• feminine: παρούσα
• neuter: παρόν

So:

• Ο επικεφαλής είναι παρών. = The male head is present.
• Η επικεφαλής είναι παρούσα. = The female head is present.

Not exactly.

• παρών means present, in attendance, not absent
• εδώ means here

A person who is παρών is present in the relevant situation, meeting, or place.
A person who is εδώ is simply here.

So παρών is often a bit more formal and is especially common in contexts like attendance, meetings, official situations, or notices.

Because παρών is a predicate adjective: it describes the subject after the verb to be.

This is the normal pattern:

• subject + είναι
  • adjective

So:

• Ο επικεφαλής είναι παρών.

That is the standard, neutral word order.

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

The neutral order is:

• Ο επικεφαλής είναι παρών.

But you could also say:

• Παρών είναι ο επικεφαλής.

This gives more emphasis to παρών.

Still, for learners, the original order is the safest and most natural one to use first.

A rough pronunciation is:

o epikefalís íne parón

Stress falls on:

• επικεφαλής
• είναι
• παρών

So the rhythm is roughly:

o e-pi-ke-fa-LIS I-ne pa-RON

You would say:

Η επικεφαλής είναι παρούσα.

Notice:

• επικεφαλής stays the same
• the article changes from ο to η
• the adjective changes from παρών to παρούσα