Κέρδισε τον διαγωνισμό φωτογραφίας.

The dictionary form is κερδίζω, meaning to win.

In this sentence, κέρδισε is the aorist form, 3rd person singular. In everyday English, that is usually translated as the simple past:

κέρδισε = he/she won

The aorist here presents the action as a single completed event.

Greek often leaves out subject pronouns when they are not needed. The verb ending already tells you the person and number.

So κέρδισε already means he/she/it won.

That means:

• Κέρδισε τον διαγωνισμό φωτογραφίας. = He/She won the photography contest.

If Greek speakers add αυτός or αυτή, it is usually for emphasis, contrast, or clarity.

Yes. On its own, κέρδισε can also be the 2nd person singular aorist imperative, meaning win!

So depending on context, κέρδισε could be:

• he/she won
• win!

In this sentence, the intended meaning is the past tense one: won. Context usually makes the meaning clear.

Τον is the masculine singular accusative definite article, meaning the.

It appears because διαγωνισμό is the direct object of the verb:

• ο διαγωνισμός = the contest when it is the subject
• τον διαγωνισμό = the contest when it is the object

Since the sentence is about winning the contest, Greek uses the accusative form τον.

Because it is in the accusative case.

The dictionary form is:

• ο διαγωνισμός = the contest

But after a verb like κερδίζω, the thing won is the direct object, so Greek uses the accusative:

• τον διαγωνισμό

This is a very common pattern with masculine nouns ending in -ος:

• ο φίλος → τον φίλο
• ο δρόμος → τον δρόμο
• ο διαγωνισμός → τον διαγωνισμό

Φωτογραφίας is the genitive singular of η φωτογραφία.

Here it tells you what kind of contest it is:

• διαγωνισμός φωτογραφίας = photography contest / photo contest

Literally, it is something like contest of photography.

English often uses:

• a noun before another noun, as in photography contest
• or an adjective, as in photographic contest

Greek very often uses a genitive noun for this kind of relationship.

Because in this kind of phrase, Greek often uses a bare genitive without an article to show type or category.

So:

• διαγωνισμός φωτογραφίας = a photography contest / photography contest

This sounds natural and idiomatic.

Using της φωτογραφίας would usually sound more specific and less natural here, almost like the contest of the photography, which is not the normal way to say photography contest.

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

The sentence as given:

• Κέρδισε τον διαγωνισμό φωτογραφίας.

is a natural way to say it.

But you could also say:

• Τον διαγωνισμό φωτογραφίας κέρδισε.

That version gives more emphasis to the photography contest.

So the words can move around, but the case endings still show what each word is doing.

Yes. Τον is the definite article, so the sentence refers to a specific contest:

• τον διαγωνισμό φωτογραφίας = the photography contest

If you wanted a photography contest, you would use an indefinite expression such as:

• Κέρδισε έναν διαγωνισμό φωτογραφίας.

So the article matters, just as it does in English.