Η σφουγγαρίστρα είναι πίσω από την πόρτα και ο κουβάς είναι κάτω από τον νεροχύτη.

Because Greek articles change according to case.

• η = feminine singular nominative article
• την = feminine singular accusative article
• ο = masculine singular nominative article
• τον = masculine singular accusative article

In this sentence:

• η σφουγγαρίστρα and ο κουβάς are the subjects, so they are in the nominative
• την πόρτα and τον νεροχύτη come after a preposition, so they are in the accusative

English does not show this kind of change in the article, but Greek does.

This is a normal masculine noun pattern in Greek.

The dictionary form is:

• ο νεροχύτης = nominative singular

After a preposition such as από, Greek uses the accusative:

• τον νεροχύτη

So the ending changes from -ης to -η.

A similar pattern is:

• ο μαθητής → τον μαθητή
• ο οδηγός → τον οδηγό
  (different noun type, but same idea: nominative and accusative can differ)

By contrast, πόρτα does not change its noun ending here:

• η πόρτα
• την πόρτα

The article changes, but the noun itself stays the same.

Because πίσω από and κάτω από are the normal ways to say behind and under/below when they are followed by a noun.

So Greek uses:

• πίσω από + accusative noun
• κάτω από + accusative noun

Examples:

• πίσω από την πόρτα
• κάτω από τον νεροχύτη

On their own, πίσω and κάτω can also work more like adverbs:

• Είναι πίσω. = It is in the back / behind.
• Είναι κάτω. = It is below / downstairs / down.

But when you specify behind what? or under what?, Greek normally uses από.

After από, Greek normally uses the accusative.

That is why you get:

• την πόρτα
• τον νεροχύτη

This is very useful to remember, because many common Greek prepositions are followed by the accusative.

In this sentence, both location expressions use από, so both nouns after it are accusative.

Yes, often it can.

The full sentence:

• Η σφουγγαρίστρα είναι πίσω από την πόρτα και ο κουβάς είναι κάτω από τον νεροχύτη.

A very natural shorter version is:

• Η σφουγγαρίστρα είναι πίσω από την πόρτα και ο κουβάς κάτω από τον νεροχύτη.

Greek often omits repeated words when the meaning is clear.
Keeping the second είναι is also completely correct and sounds clear and neutral.

So both are possible; the version with both verbs is just a bit more explicit.

Not completely. Greek word order is more flexible than English word order.

The sentence as given is very natural and neutral:

• Η σφουγγαρίστρα είναι πίσω από την πόρτα...

But Greek can move parts around for emphasis, for example:

• Πίσω από την πόρτα είναι η σφουγγαρίστρα.
• Κάτω από τον νεροχύτη είναι ο κουβάς.

These alternatives are grammatical, but they change the emphasis.
The original version is the most straightforward choice for a learner.

Greek uses the definite article very regularly with specific nouns.

Here the speaker is referring to specific things:

• η σφουγγαρίστρα
• η πόρτα
• ο κουβάς
• ο νεροχύτης

That is why the articles appear.

In many situations, Greek uses the article more often than English does, so learners should get used to saying the noun together with its article:

• ο κουβάς
• η πόρτα
• ο νεροχύτης

This is also helpful because the article tells you the noun’s gender, number, and often its case.

They are:

• η σφουγγαρίστρα — feminine
• η πόρτα — feminine
• ο κουβάς — masculine
• ο νεροχύτης — masculine

In Greek, grammatical gender is important because it affects:

• the article
• adjectives
• sometimes noun endings in different cases

That is why it is best to learn nouns together with their article, not by themselves.

Yes, it is a long word and can feel intimidating at first.

A rough pronunciation guide is:

• σφουγγαρίστρα ≈ sfoung-ga-REE-stra

The stress is on ρί.

A rough pronunciation of the whole sentence is:

• ee sfoung-ga-REE-stra EE-ne PEE-so a-PO teen POR-ta kee o koo-VAS EE-ne KA-to a-PO ton ne-ro-HEE-tee

A few helpful stress points:

• σφουγγαρίστρα
• είναι
• πίσω
• πόρτα
• κουβάς
• κάτω
• νεροχύτη

If you say the stressed syllables clearly, your pronunciation will sound much more natural.

The accent mark shows which syllable is stressed when the word is pronounced.

For example:

• σφουγγαρίστρα → stress on ρί
• κουβάς → stress on βάς
• νεροχύτη → stress on χύ
• πόρτα → stress on πόρ

In Modern Greek, stress is very important. If you ignore it, people will often still understand you, but your speech may sound unnatural, and in some cases stress can help distinguish forms.

So when learning vocabulary, it is best to learn:

• the article
• the full spelling
• the stress

For example, learn ο κουβάς, not just κουβας.

Yes. και is the normal Greek word for and.

It joins the two parts:

• Η σφουγγαρίστρα είναι πίσω από την πόρτα
• ο κουβάς είναι κάτω από τον νεροχύτη

So it works very much like English and.

It is extremely common and useful, and unlike some other words in Greek, it does not change form.