Ο γείτονας με βοηθάει όταν έχω πρόβλημα.

Ο is the masculine singular definite article, meaning the.

So:

• ο γείτονας = the neighbor

Greek articles change depending on gender, number, and case. Here, γείτονας is masculine singular and is the subject of the sentence, so ο is the correct form.

γείτονας means neighbor and here it is in the nominative singular, because it is the subject of the verb βοηθάει.

So in this sentence:

• Ο γείτονας = the neighbor = the person doing the action

The ending -ας is a very common masculine noun ending in Greek. You do not need to translate the ending by itself; it is just part of the noun’s form.

με means me.

It is a weak object pronoun (often called a clitic), and in Greek these pronouns usually go before the finite verb.

So:

• με βοηθάει = helps me

This is different from English, where me usually comes after the verb: helps me. In Greek, the normal order is:

• με βοηθάει
  not usually
• βοηθάει με

βοηθάει is the 3rd person singular present tense form of βοηθάω / βοηθώ, meaning to help.

So:

• βοηθάει = he/she/it helps

In this sentence, the subject is ο γείτονας, so it means:

• the neighbor helps me

Greek often has more than one accepted present-tense spelling/form for some verbs. For this verb, you may also see:

• βοηθάει
• βοηθά

Both are common and correct in Modern Greek.

Greek often omits subject pronouns when they are not needed, because the verb ending already shows the person.

Here:

• βοηθάει already tells you the subject is he/she/it
• έχω already tells you I have

Since the nouns or verb forms make the subject clear, Greek usually does not need pronouns like he or I unless the speaker wants emphasis.

So Greek says:

• Ο γείτονας με βοηθάει rather than something like
• Ο γείτονας αυτός με βοηθάει

And:

• όταν έχω πρόβλημα rather than
• όταν εγώ έχω πρόβλημα

όταν means when, but in a sentence like this, with the present tense, it often has the sense of whenever.

So:

• Ο γείτονας με βοηθάει όταν έχω πρόβλημα.

can naturally mean:

• My neighbor helps me when I have a problem
• My neighbor helps me whenever I have a problem

The exact English choice depends on context, but the Greek is perfectly natural for a general or repeated situation.

Because Greek is a language that commonly drops subject pronouns.

The ending -ω in έχω already tells you the subject is I.

So:

• έχω = I have

Adding εγώ is possible, but it usually adds emphasis, contrast, or clarity:

• όταν εγώ έχω πρόβλημα = when I have a problem with extra emphasis on I

In a neutral sentence, just όταν έχω πρόβλημα is more natural.

Greek often leaves out the article in expressions like this when speaking generally.

So:

• έχω πρόβλημα = I have a problem / I’m having trouble

This does not necessarily mean one specific problem known to both speakers. It is often just a general statement.

Compare:

• έχω πρόβλημα = I have a problem / I’m in trouble
• έχω ένα πρόβλημα = I have one/a particular problem
• έχω το πρόβλημα = I have the problem (specific, known problem)

In your sentence, the article is omitted because the meaning is general and natural that way.

No, Ο γείτονας βοηθάει με is not the normal form.

The pronoun με usually goes before the conjugated verb:

• Ο γείτονας με βοηθάει.

Greek word order is somewhat flexible, but weak object pronouns like με, σε, τον, την, μας normally stay in their usual position.

You could change word order for emphasis in other parts of the sentence, for example:

• Όταν έχω πρόβλημα, ο γείτονας με βοηθάει.

But με would still normally stay before βοηθάει.

γείτονας is pronounced approximately GHEE-to-nas.

A few useful points:

• γ before ε / ι sounds like a soft sound, somewhat like the y sound in yes mixed with a voiced fricative; English has no exact match.
• εί is pronounced ee
• the accent mark shows which syllable is stressed

So the stress is on the first syllable:

• ΓΕΙ-το-νας

The accent marks in Greek are important because they show stress.

βοηθάει is pronounced approximately vo-ee-THA-ee or more smoothly in natural speech.

It looks long because it contains several vowels next to each other:

• βο-η-θά-ει

The stress falls on θά:

• βοηθάει

You may also hear the shorter written form:

• βοηθά

which is very common in everyday Greek.

Normally it describes a habitual or general situation.

Because both verbs are in the present tense:

• με βοηθάει = helps me
• όταν έχω πρόβλημα = when/whenever I have a problem

the sentence usually means something like:

• My neighbor helps me whenever I have a problem

If you wanted to describe one specific past event, Greek would normally use past-tense forms instead.

In this sentence, ο γείτονας is grammatically masculine, so it refers to a male neighbor.

For a female neighbor, Greek would normally use:

• Η γειτόνισσα με βοηθάει όταν έχω πρόβλημα.

So:

• ο γείτονας = the male neighbor
• η γειτόνισσα = the female neighbor