Η τρύπα αποστράγγισης βοηθάει στην αποστράγγιση του νερού.

Because it is in the genitive singular.

Greek often uses the pattern noun + genitive noun where English would use a compound noun or an of phrase. So:

• τρύπα αποστράγγισης literally = hole of drainage
• natural English = drainage hole or drain hole

The base noun is η αποστράγγιση. In the genitive singular, it becomes της αποστράγγισης.

Because the verb βοηθάει here is followed by σε + noun, meaning helps with / helps in something.

So:

• σε την αποστράγγιση = in the drainage / with the draining
• στην αποστράγγιση is just the contracted form of σε την αποστράγγιση

This is a very common Greek pattern:

• βοηθάω σε κάτι = help with something

Στην is a contraction of:

• σε = in, to, at
• την = the (feminine accusative singular)

So:

• σε την αποστράγγιση → στην αποστράγγιση

Greek does this contraction very often:

• σε + το = στο
• σε + τη(ν) = στη(ν)

Because αποστράγγιση is a noun meaning drainage / draining, and nouns like this often take a genitive complement in Greek.

So:

• η αποστράγγιση του νερού = the drainage of the water

This is similar to other Greek noun phrases such as:

• η χρήση του εργαλείου = the use of the tool
• η κατανάλωση του νερού = the consumption of the water

So του νερού depends on αποστράγγιση, not directly on the verb.

Here is the breakdown:

• Η τρύπα — nominative singular feminine
  It is the subject of the sentence.

• αποστράγγισης — genitive singular feminine
  It modifies τρύπα: drainage hole

• στην αποστράγγιση — accusative singular feminine after σε
  This is the phrase governed by βοηθάει

• του νερού — genitive singular neuter
  It depends on αποστράγγιση: drainage of the water

Yes. In everyday Modern Greek, βοηθάει and βοηθά are both used for he/she/it helps.

So these are both possible:

• Η τρύπα αποστράγγισης βοηθάει...
• Η τρύπα αποστράγγισης βοηθά...

Βοηθάει may sound a little fuller or a bit more colloquial in some contexts, but both are standard and correct.

Because the two instances do different jobs:

• τρύπα αποστράγγισης = drainage hole
  Here it describes what kind of hole it is.

• στην αποστράγγιση του νερού = in the drainage of the water
  Here it names the process the hole helps with.

Greek often repeats the same root in technical or descriptive sentences more readily than English does. English might prefer to vary the wording, but Greek is perfectly comfortable with this repetition.

Yes, τρύπα αποστράγγισης is understandable and natural enough, especially in practical everyday language.

Depending on context, you might also see:

• οπή αποστράγγισης — more technical or formal
• τρύπα για την αποστράγγιση — more explanatory
• sometimes other context-specific terms if talking about plumbing, machines, sinks, etc.

So τρύπα αποστράγγισης is fine, but οπή αποστράγγισης can sound more technical.

Very often, yes. Greek uses the definite article quite a lot, often more naturally than English does.

In this sentence:

• Η τρύπα = the hole
• του νερού = of the water

Even when English might sound more natural with a bare noun, Greek often prefers an article. This is especially common when talking about a specific thing in context, or when the noun is treated as a concrete, known item.

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

The neutral order here is:

• Η τρύπα αποστράγγισης βοηθάει στην αποστράγγιση του νερού.

That is a straightforward subject + verb + complement structure.

You could move parts around for emphasis, but the original version is the most neutral and natural for a learner to remember.

A rough pronunciation guide is:

• Η τρύπα → ee TREE-pa
• αποστράγγισης → a-po-STRA-ngi-sis
• βοηθάει → vo-ee-THA-ee
• στην αποστράγγιση → steen a-po-STRA-ngi-si
• του νερού → too ne-ROO

The main written stress marks are on:

• τρύπα
• αποστράγγισης
• βοηθάει
• αποστράγγιση
• νερού

Pay special attention to βοηθάει, because the vowel sequence is pronounced across separate vowel sounds, not as a single simple English-style vowel.

Yes. Greek could use a more verbal structure, for example with a phrase meaning helps the water drain.

But the version you have is very typical of explanatory, technical, or instructional language, where Greek often prefers nouns such as:

• αποστράγγιση = drainage
• λειτουργία = operation
• χρήση = use

So the sentence sounds a bit formal or technical, but it is perfectly normal Greek.