Ο επαγγελματίας δουλεύει προσεκτικά.

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

In this sentence:

• Ο = the
• επαγγελματίας = professional

So Ο επαγγελματίας means the professional.

A useful point: Greek articles change depending on gender, number, and case, unlike English the, which never changes.

Greek often uses the definite article more frequently than English does. So where English might say the professional works carefully, Greek naturally says Ο επαγγελματίας δουλεύει προσεκτικά.

If you remove the article and say just επαγγελματίας, it sounds less like a complete normal statement and more like:

• a label
• a headline
• or part of a different structure

In ordinary full sentences, using the article here is the most natural choice.

επαγγελματίας can be used for both masculine and feminine, depending on the person being described.

So:

• Ο επαγγελματίας = the professional (masculine)
• Η επαγγελματίας = the professional (feminine)

The noun itself stays the same, but the article changes:

• Ο for masculine
• Η for feminine

This is something English speakers often notice because English nouns usually do not change this way.

δουλεύει is the 3rd person singular present tense form of δουλεύω (to work).

So it means:

• he works
• she works
• it works
• or, depending on context, is working

In this sentence, it matches Ο επαγγελματίας, which is singular.

A very basic pattern is:

• δουλεύω = I work
• δουλεύεις = you work
• δουλεύει = he/she/it works

Greek often does not need a subject pronoun, because the verb ending already shows the person and number.

Here, δουλεύει already tells you the subject is 3rd person singular. Since the noun Ο επαγγελματίας is also present, there is no need to add he or she.

This is very common in Greek. Compare:

• Δουλεύω = I work
  No separate I is required.
• Δουλεύει = he/she works

Greek can include pronouns for emphasis, but they are often omitted.

It can mean either one, depending on context.

Greek present tense often covers both:

• he works
• he is working

So Ο επαγγελματίας δουλεύει προσεκτικά could mean:

• The professional works carefully
• The professional is working carefully

Usually the surrounding context tells you which meaning is intended.

προσεκτικά is an adverb, meaning it describes how the action is done.

Here it describes δουλεύει (works), so it means carefully.

Compare:

• προσεκτικός = careful (masculine adjective)
• προσεκτική = careful (feminine adjective)
• προσεκτικό = careful (neuter adjective)
• προσεκτικά = carefully (adverb)

So in this sentence, the person is not being directly described as careful; instead, the sentence says that the person works carefully.

Because the sentence needs an adverb, not an adjective.

• An adjective describes a noun: a careful professional
• An adverb describes a verb: works carefully

In this sentence, προσεκτικά describes how the professional works, so the adverb is required.

If you used προσεκτικός, that would mean careful as a description of the person, not of the way they work.

Yes. Greek word order is more flexible than English word order, because endings and articles give grammatical information.

The neutral, straightforward order here is:

• Ο επαγγελματίας δουλεύει προσεκτικά.

But other orders are possible for emphasis, such as:

• Προσεκτικά δουλεύει ο επαγγελματίας.
• Ο επαγγελματίας προσεκτικά δουλεύει.

These may sound more marked or stylistically different, but they are possible.

For learners, the original order is the safest and most natural to use.

The stress falls on these syllables:

• Ο επαγγελματίας → e-pa-nge-lma-TI-as
• δουλεύει → dou-LE-vi
• προσεκτικά → pro-sekti-KA

A rough pronunciation guide is:

• Ο επαγγελματίας = o epangelma-TI-as
• δουλεύει = thou-LEH-vee or dhou-LEH-vee depending on how you represent δ
• προσεκτικά = pro-sek-tee-KA

Important pronunciation notes:

• ου sounds like oo
• ει sounds like ee
• γγ in επαγγελματίας sounds like ng before g/k-type sounds

No. δουλεύει is very common and natural, but Greek also has εργάζεται, from εργάζομαι, which can also mean works.

So you could also hear:

• Ο επαγγελματίας εργάζεται προσεκτικά.

The difference is mostly about style and usage:

• δουλεύει is very common in everyday Greek
• εργάζεται can sound a bit more formal or more specifically related to working/being employed, depending on context

For a learner, δουλεύει is an excellent verb to know and use.

Yes, that is a very good habit.

Instead of learning just:

• επαγγελματίας

it is better to learn:

• ο επαγγελματίας
• and, if useful, η επαγγελματίας

This helps you remember:

• the noun’s gender
• how it normally appears in a sentence
• and how it will agree with other words

This is especially helpful in Greek, because articles are an important part of the grammar.