Περιμένω τη συγκάτοικό μου στην είσοδο της πολυκατοικίας.

Because Greek often leaves out the subject pronoun when it is already clear from the verb ending.

Here, Περιμένω means I wait / I am waiting.
The ending -ω already tells you the subject is I, so εγώ is unnecessary unless you want extra emphasis.

So:

• Περιμένω = I wait / I am waiting
• Εγώ περιμένω = I am the one waiting or I am waiting

It is the present tense.

In Greek, the present tense can often match both English:

• I wait
• I am waiting

Which one sounds best in English depends on context. In this sentence, English will usually prefer I am waiting, but the Greek form is simply the present: περιμένω.

Τη(ν) is the feminine accusative singular form of the.

The final -ν is often dropped in modern Greek before certain consonants. Since the next word begins with σ in συγκάτοικό, it is very normal to write:

• τη συγκάτοικό μου

You may also hear or sometimes see την in some contexts, but τη here is completely standard.

Because it is the direct object of περιμένω.

The verb περιμένω takes someone or something that is being waited for, so that noun goes in the accusative case.

Compare:

• η συγκάτοικος = the roommate as the subject
• τη συγκάτοικο = the roommate as the object

So in this sentence, I am waiting for my roommate, the word must be in the accusative: τη συγκάτοικο.

Because in Greek, grammatical gender and word endings do not always match the patterns English speakers expect.

συγκάτοικος can refer to a male or female roommate, and the article usually shows the gender:

• ο συγκάτοικος = the male roommate
• η συγκάτοικος = the female roommate

In your sentence, τη συγκάτοικό μου, the article τη shows that the roommate is feminine.

So yes, a noun ending in -ος can still be feminine.

The basic accusative form is συγκάτοικο.

When a word stressed on the third syllable from the end is followed by an enclitic like μου, Greek spelling adds a second accent:

• συγκάτοικο
• συγκάτοικό μου

This extra accent helps show the correct stress pattern in pronunciation.

The same thing happens in many common phrases, for example:

• ο άνθρωπός μου
• το τηλέφωνό μου

In Greek, possessive words like μου often come after the noun.

So:

• η συγκάτοικός μου = my roommate
• literally, something like the roommate my

This is the normal Greek pattern with these weak possessive pronouns:

• μου = my
• σου = your
• του / της = his / her

Putting μου after the noun is the usual and natural choice.

Στην is a contraction of:

• σε = in, at, to
• την = the

So:

• σε την είσοδο becomes στην είσοδο

This is very common in Greek:

• στον = σε τον
• στο = σε το
• στη(ν) = σε τη(ν)

Because είσοδο begins with a vowel, the -ν is kept: στην είσοδο.

Because after σε and its contracted forms like στην, Greek uses the accusative case.

The dictionary form is:

• η είσοδος = the entrance

But after στην, it becomes accusative:

• στην είσοδο = at/in/to the entrance

So the form is not random; it is required by the preposition σε.

Because this part means of the apartment building, not the apartment building as a direct object.

Greek uses the genitive case here to show possession or relationship:

• η είσοδος της πολυκατοικίας = the entrance of the apartment building / the apartment building’s entrance

So:

• της πολυκατοικίας = genitive singular
• base form: η πολυκατοικία

This is a very common pattern in Greek:

• η πόρτα του σπιτιού = the door of the house
• το τέλος της ταινίας = the end of the film

Because Greek articles change form depending on gender, number, and case.

Here both refer to feminine singular nouns, but they are in different cases:

• τη = feminine singular accusative
• της = feminine singular genitive

So even though English just says the, Greek shows the grammatical role more clearly.

In this sentence:

• τη συγκάτοικό μου = direct object, so accusative
• της πολυκατοικίας = of the apartment building, so genitive

Greek word order is more flexible than English because case endings show what each noun is doing.

The given order is natural and neutral:

• Περιμένω τη συγκάτοικό μου στην είσοδο της πολυκατοικίας.

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

• Στην είσοδο της πολυκατοικίας περιμένω τη συγκάτοικό μου.
• Τη συγκάτοικό μου περιμένω στην είσοδο της πολυκατοικίας.

These alternatives are possible, but the original sentence is the most straightforward and neutral version.