Όταν δεν βρέχει, ποτίζω το παρτέρι μπροστά από το σπίτι.

Όταν can mean when, but in sentences like this one it often has the sense of whenever.

So:

• Όταν δεν βρέχει, ποτίζω το παρτέρι... = When/Whenever it doesn’t rain, I water the flower bed...

Because the main verb ποτίζω is in the present tense and describes a habitual action, English often translates this more naturally as whenever it doesn’t rain.

Δεν is the normal word for not in Modern Greek.

It usually goes directly before the verb:

• δεν βρέχει = it isn’t raining / it doesn’t rain

This is very typical Greek word order for negation:

• γράφω = I write
• δεν γράφω = I do not write

So Όταν δεν βρέχει literally has the structure when not rains.

Greek, like English, treats weather verbs as impersonal in many cases.

• βρέχει = it rains / it is raining

There is no separately stated subject, just as English uses it in a non-literal way. Greek normally does not need to add a word corresponding to that English it.

So:

• βρέχει = it rains
• δεν βρέχει = it doesn’t rain

Because the sentence describes a habitual or general action, not a one-time event.

• βρέχει = it rains / it is raining
• ποτίζω = I water / I am watering

In this context, Greek present tense often expresses what someone usually does:

• Όταν δεν βρέχει, ποτίζω... = When it doesn’t rain, I water... or more naturally, Whenever it doesn’t rain, I water...

So this is not specifically about what is happening right now. It is about a repeated pattern.

Ποτίζω is the present/imperfective form, which fits a repeated or ongoing action.

• ποτίζω = I water / I am watering / I usually water

ποτίσω is related to the perfective stem and is used in different structures, especially with future or subjunctive markers:

• θα ποτίσω = I will water
• να ποτίσω = that I water / to water once

Since this sentence describes a regular habit, ποτίζω is the correct choice.

Greek often leaves out subject pronouns because the verb ending already shows the person.

• ποτίζω = I water

The ending -ω tells you the subject is I, so εγώ is unnecessary unless you want emphasis or contrast.

For example:

• Ποτίζω το παρτέρι. = I water the flower bed.
• Εγώ ποτίζω το παρτέρι. = I water the flower bed.
  (perhaps contrasting with someone else)

So in your sentence, omitting εγώ is normal and natural.

Greek uses the definite article very often, sometimes more often than English does.

• το παρτέρι = the flower bed

In this sentence, the speaker probably means a specific flower bed, presumably the one at the house. So the definite article sounds natural.

Greek commonly says:

• πάω στο σπίτι = I’m going home / to the house
• ποτίζω το παρτέρι = I water the flower bed

Even when English sometimes sounds more general or idiomatic without emphasizing the, Greek often keeps the article.

Μπροστά από is a fixed prepositional expression meaning in front of.

It is best learned as a whole unit:

• μπροστά από το σπίτι = in front of the house

Although από often means from, in many expressions it helps form meanings that do not translate word-for-word. So here you should not try to translate each piece too literally. Just remember:

• μπροστά από = in front of
• πίσω από = behind
• δίπλα σε = next to

This is a very common pattern in Greek.

After από, Greek uses the accusative case.

In this sentence:

• μπροστά από το σπίτι

the noun phrase το σπίτι is in the accusative.

The reason it may not look different is that many neuter nouns have the same form in the nominative and accusative singular:

• nominative: το σπίτι
• accusative: το σπίτι

So yes, it is changing case grammatically, but the form happens to stay the same.

The comma separates the subordinate clause from the main clause.

• Όταν δεν βρέχει, = when it doesn’t rain
• ποτίζω το παρτέρι... = I water the flower bed...

This is similar to English, where a comma is often used when a when-clause comes first:

• When it doesn’t rain, I water the flower bed.

So the punctuation is normal and helpful.

Yes, Greek word order is fairly flexible, though some orders are more neutral than others.

The most neutral version here is:

• Όταν δεν βρέχει, ποτίζω το παρτέρι μπροστά από το σπίτι.

But you could move parts around for emphasis, for example:

• Ποτίζω το παρτέρι μπροστά από το σπίτι όταν δεν βρέχει.

This still means the same thing, but it sounds a little different in rhythm and focus. The original version is especially natural because it presents the condition first: when it doesn’t rain...

In this sentence, it most naturally describes το παρτέρι:

• the flower bed in front of the house

So the meaning is:

• I water the flower bed that is in front of the house.

Technically, context always helps, but the default reading is that the location phrase belongs with παρτέρι, not with the action of watering.

A simple pronunciation guide would be:

• Όταν ≈ OH-tan
• δεν ≈ then / thenn
• βρέχει ≈ VRE-khee
• ποτίζω ≈ po-TEE-zo
• το παρτέρι ≈ to par-TE-ri
• μπροστά από ≈ bro-STA a-PO
• το σπίτι ≈ to SPEE-ti

The stress falls on:

• Όταν
• βρέχει
• ποτίζω
• παρτέρι
• μπροστά
• από
• σπίτι

A full rough pronunciation:

OH-tan then VRE-khee, po-TEE-zo to par-TE-ri bro-STA a-PO to SPEE-ti.

Yes, but then Greek would normally change the main verb structure.

For a future meaning, you would usually say:

• Όταν δεν βρέχει, θα ποτίζω...
  if you mean a repeated future habit in some context

or more commonly in a specific future situation, Greek often uses different wording depending on exactly what is meant.

Your original sentence with simple present tense most naturally expresses a general present habit:

• Whenever it doesn’t rain, I water the flower bed.

So the original is best understood as a habitual statement, not a specific future plan.