Ο χάρακας είναι πάνω στο βιβλίο, αλλά ο μαρκαδόρος είναι μέσα στην κασετίνα.

Because Greek articles change according to both gender and grammatical role.

In this sentence:

• ο χάρακας = the ruler
• ο μαρκαδόρος = the marker

Here, ο is the masculine singular nominative article, used because these nouns are the subjects of the sentence.

But after prepositions like πάνω σε and μέσα σε, Greek normally uses the accusative:

• στο βιβλίο = on the book
• στην κασετίνα = in the pencil case

So:

• το βιβλίο is neuter → after σε, σε + το = στο
• την κασετίνα is feminine accusative → σε + την = στην

So the different article forms are not random; they reflect case and gender.

στο is a contraction of:

• σε = in / on / at / to
• το = the (neuter singular accusative)

So:

• σε το → στο

This contraction is standard Greek.

Similarly:

• σε την → στην
• σε τον → στον

So in this sentence:

• πάνω στο βιβλίο literally contains σε + το βιβλίο
• μέσα στην κασετίνα literally contains σε + την κασετίνα

They often function like location words meaning:

• πάνω = on / on top / above
• μέσα = inside / in

In everyday Greek, they are very often followed by σε + article + noun when you specify the place:

• πάνω στο βιβλίο = on the book
• μέσα στην κασετίνα = inside the pencil case

You can think of these patterns as:

• πάνω σε κάτι = on something
• μέσα σε κάτι = inside something

In natural speech, πάνω στο and μέσα στην are extremely common.

Yes, Greek often repeats the verb, and that is completely normal here:

• Ο χάρακας είναι πάνω στο βιβλίο, αλλά ο μαρκαδόρος είναι μέσα στην κασετίνα.

Repeating είναι makes the sentence clear and balanced.

You could also hear a shorter version in casual speech:

• Ο χάρακας είναι πάνω στο βιβλίο, αλλά ο μαρκαδόρος μέσα στην κασετίνα.

That kind of omission is possible because the meaning is easy to recover, but for learners the full version with both είναι forms is the safest and clearest model.

The sentence contains both nominative and accusative forms.

Nominative: subjects

• ο χάρακας
• ο μαρκαδόρος

These are the things being talked about, so they are in the nominative.

Accusative: after the prepositional expressions

• στο βιβλίο
• στην κασετίνα

After σε and expressions built with it, Greek uses the accusative.

So a helpful way to view the structure is:

• Subject
  • είναι
    • location
• ο χάρακας
  • είναι
    • πάνω στο βιβλίο
• ο μαρκαδόρος
  • είναι
    • μέσα στην κασετίνα

The nouns are:

• χάρακας = masculine
• μαρκαδόρος = masculine
• βιβλίο = neuter
• κασετίνα = feminine

You can tell partly from the article:

• ο usually marks masculine singular nominative
• το usually marks neuter singular
• η / την usually marks feminine singular

So the dictionary forms would be:

• ο χάρακας
• ο μαρκαδόρος
• το βιβλίο
• η κασετίνα

The endings also often help:

• -ας, -ος are often masculine
• -ο is often neuter
• -α is often feminine

These are common patterns, though not absolute rules.

Because after σε Greek uses the accusative, not the nominative.

The noun’s basic form is:

• η κασετίνα = the pencil case

But after the preposition, it becomes:

• σε την κασετίνα → στην κασετίνα

So:

• η = nominative feminine singular
• την = accusative feminine singular

For this noun, the noun itself stays κασετίνα in both cases, but the article changes from η to την.

Yes, but not a big one in meaning.

• πάνω is the very common everyday form.
• επάνω is a fuller form and may sound a bit more formal, emphatic, or literary in some contexts.

In ordinary speech, πάνω στο βιβλίο is exactly what you would expect.

So for most learners, πάνω is the most useful default form.

Here είναι is the 3rd person singular form of to be, so it means is:

• Ο χάρακας είναι... = The ruler is...
• ο μαρκαδόρος είναι... = the marker is...

But είναι can also mean are, depending on the subject:

• Οι χάρακες είναι... = The rulers are...

So είναι is used for both singular and plural in the present tense:

• he/she/it is
• they are

That is different from English, where is and are are separate forms.

Because αλλά means but, and Greek usually places a comma before it when it connects two clauses.

So this punctuation is very normal:

• ..., αλλά ...

It works much like English:

• The ruler is on the book, but the marker is in the pencil case.

You may occasionally see punctuation used more loosely in informal writing, but the comma here is standard.

A rough pronunciation guide:

• Ο χάρακας → o HA-ra-kas
• είναι → EE-ne
• πάνω → PA-no
• στο βιβλίο → sto viv-LEE-o
• αλλά → a-LA
• ο μαρκαδόρος → o mar-ka-THO-ros
• μέσα → ME-sa
• στην κασετίνα → stin ka-se-TEE-na

A few useful notes:

• χ in χάρακας is not like English h exactly; it is a rougher sound, like the ch in German Bach.
• δ in μαρκαδόρος is pronounced like the th in this, not like English d.
• Greek stress matters a lot, so pay attention to the written accent marks:
  • χάρακας
  • πάνω
  • βιβλίο
  • μαρκαδόρος
  • μέσα
  • κασετίνα

Yes, Greek has more flexible word order than English, though not all versions sound equally natural in every context.

The sentence as given is very natural:

• Ο χάρακας είναι πάνω στο βιβλίο, αλλά ο μαρκαδόρος είναι μέσα στην κασετίνα.

You could also hear variations for emphasis, such as:

• Πάνω στο βιβλίο είναι ο χάρακας...
• Μέσα στην κασετίνα είναι ο μαρκαδόρος...

But these feel more marked, often because the speaker wants to emphasize the location.

For learners, the safest basic pattern is:

• subject + είναι + location

More literally, it is inside the pencil case.

• μέσα = inside
• στην κασετίνα = in the pencil case

But in many everyday contexts, English simply says in the pencil case, and that is the natural translation.

So the Greek expression is slightly more explicit about the idea of being inside, but in normal usage it matches English in very well.