Η ενοικιάστρια στο διπλανό διαμέρισμα είναι φιλική και μου είπε ότι οι περισσότεροι ενοικιαστές εδώ είναι ήσυχοι.

Because the sentence talks about:

• η ενοικιάστρια = the female tenant
• οι ενοικιαστές = the tenants

Greek nouns change form for gender and number.

• ενοικιάστρια is feminine singular
• ενοικιαστές is masculine plural

A very common pattern is:

• masculine singular: ενοικιαστής
• feminine singular: ενοικιάστρια
• masculine plural: ενοικιαστές

Also, in Greek, the masculine plural is often used for a mixed group or when the gender of the group is not specified, so οι περισσότεροι ενοικιαστές can mean most tenants in general, not only male tenants.

η is the feminine singular definite article, so it means the.

Greek articles change depending on gender, number, and case. In this sentence you see:

• η ενοικιάστρια = the female tenant
• οι περισσότεροι ενοικιαστές = the / most tenants

So unlike English, Greek does not use one single form of the for everything.

Because στο is the normal contracted form of σε + το.

• σε = in / at / to
• το = the for a neuter singular noun

So:

• σε το διαμέρισμα becomes στο διαμέρισμα

This contraction is extremely common in modern Greek:

• σε + το = στο
• σε + την = στην
• σε + τον = στον

So στο διπλανό διαμέρισμα means in the next apartment or at the apartment next door, depending on context.

διπλανό is an adjective meaning next, neighboring, or adjacent. It changes form to match the noun it describes.

Here it describes διαμέρισμα, which is:

• neuter
• singular

So the adjective must also be neuter singular:

• διπλανός = masculine
• διπλανή = feminine
• διπλανό = neuter

Because διαμέρισμα is neuter, Greek uses διπλανό διαμέρισμα.

Greek nouns have grammatical gender, and διαμέρισμα happens to be neuter. This is just something you learn with the noun.

A useful clue is that many nouns ending in -μα are neuter:

• το διαμέρισμα = the apartment
• το γράμμα = the letter
• το πρόβλημα = the problem

Because διαμέρισμα is neuter, the article and adjective around it also take neuter forms:

• στο διπλανό διαμέρισμα

Because φιλική describes η ενοικιάστρια, which is feminine singular.

Greek adjectives agree with the noun they describe in gender, number, and usually case.

So:

• φιλικός = masculine singular
• φιλική = feminine singular
• φιλικό = neuter singular

Since the subject is a female tenant, Greek uses φιλική.

Because μου means to me, while με means me as a direct object.

Here the verb is λέω / είπα in the sense of say / tell, and the person receiving the information is an indirect object:

• μου είπε = she told me
• literally: to me she said

So:

• μου = to me
• με = me

English often just says told me, but Greek shows the difference more clearly.

είπε is the aorist form of λέω, and here it means said or told.

This is one of the most common irregular verbs in Greek:

• λέω = I say / I am saying
• είπα = I said
• είπε = he/she/it said

So μου είπε ότι... means she told me that...

Even though the dictionary form is λέω, the past forms come from a different stem, which is why είπε may look surprising at first.

ότι introduces a clause and means that:

• μου είπε ότι... = she told me that...

It works very much like English that in reported speech.

In everyday Greek, speakers sometimes omit it, just as English often does:

• μου είπε ότι οι περισσότεροι ενοικιαστές εδώ είναι ήσυχοι
• μου είπε οι περισσότεροι ενοικιαστές εδώ είναι ήσυχοι

But using ότι is very normal and clear.

Greek often uses the article with περισσότεροι in this kind of structure.

• οι περισσότεροι ενοικιαστές = most tenants
• more literally: the most tenants, but that is not how it is understood in English

This is a very common Greek pattern:

• οι περισσότεροι άνθρωποι = most people
• οι περισσότερες μέρες = most days
• τα περισσότερα παιδιά = most children

Here περισσότεροι agrees with ενοικιαστές, so it is masculine plural nominative.

Because in modern Greek, the verb είμαι has the same form είναι for both:

• he/she/it is
• they are

So in this sentence:

• η ενοικιάστρια ... είναι φιλική = the tenant ... is friendly
• οι περισσότεροι ενοικιαστές ... είναι ήσυχοι = most tenants ... are quiet

The subject tells you whether it is singular or plural.

Because ήσυχοι describes οι περισσότεροι ενοικιαστές, which is masculine plural.

The adjective ήσυχος changes form:

• ήσυχος = masculine singular
• ήσυχη = feminine singular
• ήσυχο = neuter singular
• ήσυχοι = masculine plural

So οι περισσότεροι ενοικιαστές είναι ήσυχοι means most tenants are quiet.

Again, the adjective must agree with the noun it describes.

Greek word order is more flexible than English word order.

Here:

• οι περισσότεροι ενοικιαστές εδώ είναι ήσυχοι

means most tenants here are quiet.

The word εδώ modifies the noun phrase οι περισσότεροι ενοικιαστές, so it means the tenants here.

Greek could also move εδώ around in some contexts, for example:

• εδώ οι περισσότεροι ενοικιαστές είναι ήσυχοι
• οι περισσότεροι ενοικιαστές είναι ήσυχοι εδώ

But these versions can shift the emphasis slightly. In your sentence, εδώ most naturally means here, in this building / in this place and stays close to ενοικιαστές.

Yes, literally it is in the next / neighboring apartment.

But in natural English, this often becomes:

• in the apartment next door
• in the next apartment
• next door

So η ενοικιάστρια στο διπλανό διαμέρισμα is naturally understood as the female tenant in the apartment next door.

Then the beginning would change to masculine:

• Ο ενοικιαστής στο διπλανό διαμέρισμα είναι φιλικός και μου είπε ότι οι περισσότεροι ενοικιαστές εδώ είναι ήσυχοι.

Changes:

• Η → Ο
• ενοικιάστρια → ενοικιαστής
• φιλική → φιλικός

This is a good example of how Greek marks gender more visibly than English.

Yes, definitely. In Greek, the written accent mark shows which syllable is stressed, and that is important for correct pronunciation.

For example:

• ενοι-κι-ά-στρια
• δι-πλα-νό
• ή-συ-χοι

If you ignore the stress marks, your Greek may still be understandable sometimes, but it will sound much less natural, and in some cases stress can help distinguish forms.

Yes, it sounds natural and idiomatic.

It contains several very common Greek patterns:

• article + noun agreement: η ενοικιάστρια
• σε + article contraction: στο
• adjective agreement: φιλική, διπλανό, ήσυχοι
• clitic object pronoun: μου είπε
• reported speech with ότι
• article with περισσότεροι: οι περισσότεροι ενοικιαστές

So it is a very useful sentence for seeing how everyday Greek grammar fits together.