Η πλαστελίνη είναι πάνω στο τραπέζι, αλλά οι νερομπογιές είναι ακόμα ανοιχτές δίπλα στο μπλοκ.

In Greek, nouns are very often used with a definite article when talking about a specific thing. So η πλαστελίνη is the natural way to say the plasticine/modeling clay.

Here, η tells you that πλαστελίνη is:

• feminine
• singular
• nominative (the subject of the clause)

Greek uses articles more often than English, so leaving the article out would usually sound less natural here.

Because in Modern Greek, είναι is the same form for:

• he/she/it is
• they are

So:

• Η πλαστελίνη είναι... = The plasticine is...
• Οι νερομπογιές είναι... = The watercolors are...

English changes between is and are, but Greek does not in the 3rd person present tense.

πάνω στο τραπέζι means on the table or more literally on top of the table.

It is made of:

• πάνω = on top / above
• στο = σε + το
• τραπέζι = table

So the structure is basically:

• πάνω σε το τραπέζι → πάνω στο τραπέζι

In normal speech and writing, σε + το contracts to στο.

After σε, Greek normally uses the accusative case. But with many neuter nouns, the nominative and accusative forms are the same.

So:

• το τραπέζι = the table
• after σε: στο τραπέζι

You do not see a change in the noun itself here, even though the case function has changed.

Οι νερομπογιές is the plural of η νερομπογιά.

It refers to watercolor paints / watercolors, usually understood as a set of paints. That is very natural in Greek.

So:

• η νερομπογιά = one watercolor paint
• οι νερομπογιές = watercolors / watercolor paints

The article οι shows that the noun is:

• feminine
• plural
• nominative

Because ανοιχτές is an adjective agreeing with νερομπογιές.

In Greek, adjectives agree with the noun they describe in:

• gender
• number
• case

Since νερομπογιές is feminine plural, the adjective must also be feminine plural:

• ανοιχτός = masculine singular
• ανοιχτή = feminine singular
• ανοιχτό = neuter singular
• ανοιχτές = feminine plural

So οι νερομπογιές είναι ανοιχτές = the watercolors are open.

Here ακόμα means still.

So:

• είναι ακόμα ανοιχτές = are still open

That suggests they were already open before, and they remain open now.

Be aware that ακόμα can have other meanings in other contexts, such as even or any more, but in this sentence the natural meaning is still.

δίπλα στο μπλοκ means next to the pad/block.

It is built like this:

• δίπλα = beside / next to
• στο = σε + το
• μπλοκ = pad, notepad, sketchpad, writing pad, depending on context

So literally it is beside the pad.

Just like before:

• σε το μπλοκ → στο μπλοκ

μπλοκ is a loanword, and in Modern Greek it is usually treated as a neuter noun.

In many everyday uses, it looks the same in the forms you are most likely to meet, especially here:

• το μπλοκ
• στο μπλοκ

So even though it is part of a case structure, the visible form often stays the same. That is normal for this noun.

Because Greek uses definite articles much more regularly than English.

In this sentence, all the nouns are presented as specific, identifiable things:

• η πλαστελίνη
• το τραπέζι
• οι νερομπογιές
• το μπλοκ

In English, you might sometimes omit an article in similar contexts, but Greek usually sounds more natural with it.

Yes. Η πλαστελίνη είναι στο τραπέζι is perfectly natural and usually means the plasticine is on the table.

Adding πάνω makes the location a bit more explicit:

• στο τραπέζι = on/at the table, depending on context
• πάνω στο τραπέζι = clearly on top of the table

So πάνω adds emphasis and precision.

Not completely. Greek word order is more flexible than English, because endings and articles help show grammatical relationships.

The sentence as written is neutral and natural:

• Η πλαστελίνη είναι πάνω στο τραπέζι, αλλά οι νερομπογιές είναι ακόμα ανοιχτές δίπλα στο μπλοκ.

But parts can move for emphasis. For example:

• Πάνω στο τραπέζι είναι η πλαστελίνη...
• Οι νερομπογιές είναι δίπλα στο μπλοκ ακόμα ανοιχτές...

Those alternatives may sound more marked or emphasize different parts, but they are possible. The original version is the most straightforward for a learner.