Σήμερα το πεπόνι είναι πολύ γλυκό, αλλά το καρπούζι είναι πιο καλό όταν είναι κρύο.

Το is the definite article in the neuter singular, equivalent to the.

Here it appears because both πεπόνι and καρπούζι are neuter nouns:

• το πεπόνι = the melon
• το καρπούζι = the watermelon

In Greek, the definite article is used much more often than in English. It can refer to:

• a specific thing, or
• a whole category in a general statement

So this sentence can sound like it is talking about these fruits as types of fruit, not necessarily one particular melon and one particular watermelon.

Because they are adjectives agreeing with neuter singular nouns.

Both πεπόνι and καρπούζι are neuter singular, so the adjectives describing them must also be neuter singular:

• το πεπόνι είναι γλυκό
• το καρπούζι είναι καλό
• το καρπούζι είναι κρύο

This is a basic rule in Greek: adjectives agree with the noun in gender, number, and case.

For comparison:

• masculine: καλός
• feminine: καλή
• neuter: καλό

Σήμερα means today.

It is placed at the beginning because Greek word order is fairly flexible, and putting σήμερα first gives it a natural time-setting role:

• Σήμερα το πεπόνι είναι πολύ γλυκό = Today the melon is very sweet

You could move it, and the sentence would still be grammatical, for example:

• Το πεπόνι σήμερα είναι πολύ γλυκό

But starting with σήμερα is very normal and often sounds more natural when introducing the time frame first.

Πολύ here means very and works as an adverb modifying the adjective γλυκό.

So:

• πολύ γλυκό = very sweet

This is similar to English:

• sweet → adjective
• very sweet → adverb + adjective

In Greek, πολύ can mean:

• much / a lot
• very

The exact meaning depends on what it modifies.

Both are possible ways to express the comparative better.

• πιο καλό = literally more good
• καλύτερο = the single-word comparative better

So:

• το καρπούζι είναι πιο καλό όταν είναι κρύο
• το καρπούζι είναι καλύτερο όταν είναι κρύο

Both are understandable. In many contexts, καλύτερο sounds more idiomatic or more standard to learners because καλός has the irregular comparative καλύτερος / καλύτερο. But πιο + adjective is also a very common Greek comparative pattern.

Because Greek uses είναι for is in each clause where English also uses a form of to be.

The sentence has three parts:

• το πεπόνι είναι πολύ γλυκό
• το καρπούζι είναι πιο καλό
• όταν είναι κρύο

So είναι appears each time the verb is is needed.

Greek does not usually drop the verb to be in this kind of sentence. Each clause still needs its verb.

The understood subject is το καρπούζι.

So όταν είναι κρύο means:

• when it is cold

Greek often omits subject pronouns such as it, he, she, I, we, because the verb form and the context usually make the subject clear.

Here, the listener naturally understands:

• το καρπούζι είναι πιο καλό όταν είναι κρύο
• watermelon is better when it is cold

Also, κρύο is neuter singular, matching καρπούζι.

In sentences like this, yes: όταν means when and introduces a time clause.

Here:

• όταν είναι κρύο = when it is cold

It tells you the condition in time under which the previous statement is true. In other words, the watermelon is better in the situation where it is cold.

It can be understood either way depending on context, but Greek often uses the singular with the definite article for general statements.

So:

• το πεπόνι can mean melon as a category
• το καρπούζι can mean watermelon as a category

That means the sentence can naturally be understood as a general observation:

• melon is very sweet today, but watermelon is better when cold

If the surrounding context were about actual fruit on the table, it could also refer to specific fruit. Greek leaves that to context more often than English does.

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

The original sentence is natural:

• Σήμερα το πεπόνι είναι πολύ γλυκό, αλλά το καρπούζι είναι πιο καλό όταν είναι κρύο.

But other orders are possible, depending on emphasis:

• Το πεπόνι είναι πολύ γλυκό σήμερα
• Το καρπούζι είναι πιο καλό όταν είναι κρύο
• Όταν είναι κρύο, το καρπούζι είναι πιο καλό

The meaning stays mostly the same, but the focus can shift slightly depending on what the speaker wants to emphasize.

Not necessarily.

In English, better often suggests a comparison with something else. In this sentence, the comparison is really between:

• watermelon at room temperature, and
• watermelon when cold

So το καρπούζι είναι πιο καλό όταν είναι κρύο means:

• watermelon is better when it is cold

It does not automatically mean:

• watermelon is better than the melon

If Greek wanted to make that comparison explicit, it would usually say so more directly.