Η καταμέτρηση αρχίζει αύριο.

Because it is not the English H. It is the Greek capital letter eta (Η / η), and in Modern Greek it is pronounced ee /i/.

In this sentence, Η is the feminine singular form of the.

A very useful distinction:

• Η = the (feminine article)
• ή = or

The accent mark matters.

Η is the definite article: the.

It goes with καταμέτρηση, so η καταμέτρηση means the counting / the count.

Grammatically, it matches the noun in:

• gender: feminine
• number: singular
• case: nominative

Together, η καταμέτρηση is the subject of the sentence.

The clearest clue here is the article η, which marks it as feminine.

Also, many Greek nouns ending in -ηση are feminine, so that ending is a helpful pattern to learn.

So the dictionary-style form would be:

• η καταμέτρηση

not ο καταμέτρηση or το καταμέτρηση.

It is a noun meaning something like:

• counting
• count
• tally
• enumeration

The best English translation depends on context. For example, it could refer to:

• counting votes
• counting items
• taking stock
• a formal tally

So in this sentence it could be understood as the count starts tomorrow or the counting begins tomorrow, depending on the situation.

Αρχίζει is the 3rd person singular present tense of αρχίζω = to begin / to start.

So:

• αρχίζω = I begin
• αρχίζεις = you begin
• αρχίζει = he/she/it begins

Since η καταμέτρηση is singular, the verb is singular too: αρχίζει.

Greek can use the present tense for a scheduled or expected future event, just like English sometimes can:

• The meeting starts tomorrow.
• Η καταμέτρηση αρχίζει αύριο.

So this is completely natural Greek.

You could also say:

• Η καταμέτρηση θα αρχίσει αύριο.

That version makes the future more explicit. Both are correct, but the present tense often sounds natural for planned events.

Here αύριο is an adverb meaning tomorrow.

That is why it appears without an article or preposition.

So:

• αρχίζει αύριο = starts tomorrow

In other contexts, αύριο can also act like a noun, especially in expressions such as το αύριο = the future / tomorrow, but in this sentence it is simply an adverb of time.

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

The sentence as given:

• Η καταμέτρηση αρχίζει αύριο.

is a neutral, straightforward order.

But these are also possible:

• Αύριο αρχίζει η καταμέτρηση.
• Αρχίζει αύριο η καταμέτρηση.

They mean basically the same thing, but the emphasis shifts:

• starting with Αύριο highlights tomorrow
• starting with Αρχίζει can sound a bit more dramatic or formal

A useful approximation for an English speaker is:

• ee ka-ta-ME-tri-si ar-HEE-zi AV-rio

More carefully:

• Η = ee
• καταμέτρηση = ka-ta-ME-tri-si
• αρχίζει = ar-HI-zi or ar-KHEE-zi as an approximation
• αύριο = AV-rio

A few pronunciation notes:

• χ in αρχίζει is not like English ch in chair
• it is a Greek throat/fricative sound
• αυ in αύριο is pronounced av here

Because αυ changes pronunciation depending on what follows.

In Modern Greek:

• αυ → av before vowels and voiced consonants
• αυ → af before voiceless consonants

Since ρ is voiced, αύριο is pronounced AV-rio.

This same rule applies in many other Greek words with αυ.

The accent marks show which syllable is stressed.

So:

• καταμέτρηση
• αρχίζει
• αύριο

This is very important in Greek pronunciation.

Notice that the article Η has no accent, because articles are normally unstressed.

Normally, no—not in ordinary standard Greek.

Greek uses the definite article much more regularly than English does, so η καταμέτρηση sounds natural, while καταμέτρηση on its own would sound unusual in a normal sentence.

Without the article, it might sound like:

• a headline
• a note
• telegraphic language

So for normal speech and writing, Η καταμέτρηση αρχίζει αύριο is the natural form.