Η πρώτη όπτηση τελειώνει αύριο.

Η is the feminine singular definite article, meaning the.

It appears as Η here only because it is the first word of the sentence and is capitalized. Normally it is written η.

It agrees with όπτηση, which is a feminine singular noun.

Because πρώτη has to agree with the noun όπτηση in gender, number, and case.

Here:

• όπτηση is feminine
• it is singular
• it is the subject of the sentence, so it is in the nominative

So the adjective πρώτος changes to the feminine nominative singular form πρώτη.

Very roughly:

• πρώτος = masculine
• πρώτη = feminine
• πρώτο = neuter

Όπτηση is a noun. In grammar terms, it is:

• feminine
• singular
• nominative here, because it is the subject of the verb

A learner may notice that nouns ending in -ηση are often feminine in Greek, and όπτηση follows that pattern.

Τελειώνει is the third-person singular present tense form of τελειώνω.

So it matches the singular subject Η πρώτη όπτηση.

You can think of it as:

• I finish / end = τελειώνω
• he/she/it finishes / ends = τελειώνει

In this sentence, the subject is not a person but a process/event, so τελειώνει means something like ends or finishes.

Because Greek often uses the present tense for a planned, expected, or scheduled future event, especially when there is a time word like αύριο (tomorrow).

English can do the same:

• The course ends tomorrow.
• The train leaves at six.

So τελειώνει αύριο is very natural Greek.

Yes. You could also say:

Η πρώτη όπτηση θα τελειώσει αύριο.

That is also correct and clearly future.

The difference is roughly this:

• τελειώνει αύριο = present used for a scheduled/expected future event
• θα τελειώσει αύριο = explicit future

Both are possible, but the original sentence sounds quite natural.

Because the sentence means that the process itself ends. The subject is η πρώτη όπτηση, and it is doing the action in a grammatical sense.

So Greek uses the active form:

• Η πρώτη όπτηση τελειώνει = The first firing ends / is finishing

A passive form would suggest something different and would not be the normal choice here.

The phrase Η πρώτη όπτηση is in the nominative case, because it is the subject of the sentence.

So:

• Η = nominative feminine singular article
• πρώτη = nominative feminine singular adjective
• όπτηση = nominative feminine singular noun

αύριο is an adverb, so it does not change for case.

Yes. Greek word order is more flexible than English, although some orders sound more neutral than others.

The original:

• Η πρώτη όπτηση τελειώνει αύριο.

You could also say:

• Αύριο τελειώνει η πρώτη όπτηση.
• Τελειώνει αύριο η πρώτη όπτηση.

These are still grammatical, but they may shift the emphasis slightly. The original version is a very neutral, straightforward order.

It is pronounced roughly AV-rio, with the stress on the first syllable: αύριο.

A useful point: αυ in Greek is pronounced:

• av before a voiced sound or a vowel
• af before a voiceless sound

Since ρ is a voiced sound, αύριο is pronounced with av, not af.

The written accents show you exactly where the stress goes:

• Η πρώτη
• όπτηση
• τελειώνει
• αύριο

Greek stress is important and can sometimes distinguish forms, so the accent marks are not optional in normal writing.

Here it is intransitive.

That means it does not take a direct object. The sentence is saying that something comes to an end.

Compare:

• Τελειώνω το βιβλίο. = I finish the book.
  Here the verb is transitive.
• Η πρώτη όπτηση τελειώνει αύριο. = The first firing ends tomorrow.
  Here the verb is intransitive.

Greek τελειώνω can work both ways, and this sentence is a good example of the intransitive use.