Learn Hardcore Greek: Ο κεντρικός σταθμός του τρένου είναι δίπλα στο κτίριο όπου δουλεύει ο φίλος μου. - The main train station is next to the building where my friend works.

Ο σταθμός του τρένου literally: “the station of the train” → idiomatically: “the train station”.
Greek normally doesn’t put two nouns side by side like English “train station”; instead it uses [noun] + του/της/του + [noun].

So το τρένο (nominative) would be wrong here; you need του τρένου to express that the station is the station of the train.

Why is κεντρικός before σταθμός, and what exactly does it mean?

Κεντρικός is an adjective meaning central, main, or downtown (depending on context). It comes before the noun and agrees with it:

ο κεντρικός σταθμός
- ο: masculine, singular, nominative
- κεντρικός: masculine, singular, nominative
- σταθμός: masculine, singular, nominative

Greek adjectives can appear either before or after the noun, but the basic, neutral pattern for a descriptive adjective is:

article + adjective + noun → ο κεντρικός σταθμός = the central station.

What is δίπλα doing here, and why do we need στο after it?

Δίπλα means next to / beside. In Greek it normally combines with the preposition σε to show what something is next to:

δίπλα σε = next to (literally “beside at/to”)
δίπλα στο κτίριο = next to the building

Στο here is the contraction of σε + το (to/at + the):

σε + το κτίριο → στο κτίριο

You cannot say δίπλα το κτίριο; you need δίπλα σε + article: δίπλα στο κτίριο.

What exactly is στο and how is it formed?

Στο is a contraction of:

σε (a very common preposition: to, at, in, on)
το (neuter singular definite article: the)

So:

σε + το κτίριο → στο κτίριο = at/to/in the building (context decides)

Similarly:

σε + τον σταθμό → στον σταθμό
σε + την πόλη → στην πόλη

In this sentence, στο κτίριο with δίπλα is understood as “next to the building”.

What does όπου mean here, and how is it different from που?

Όπου is a relative adverb meaning where. It introduces a clause that describes a place:

το κτίριο όπου δουλεύει ο φίλος μου
= the building where my friend works

Differences from που:

Όπου specifically refers to place (where).
Που is a very general relative word, often like that/which/who:
- το κτίριο που βλέπεις = the building that you see

In many everyday contexts, Greeks might also say:

το κτίριο που δουλεύει ο φίλος μου

But όπου is more clearly “where”, highlighting the location of the action.

Could I say όπου ο φίλος μου δουλεύει instead of όπου δουλεύει ο φίλος μου?

Yes, όπου ο φίλος μου δουλεύει is grammatically correct, but the neutral, most natural order in modern Greek is:

όπου δουλεύει ο φίλος μου

Greek word order is relatively flexible, but:

[όπου] + [verb] + [subject] is very common and sounds more natural here.
όπου ο φίλος μου δουλεύει might sound a bit more marked or emphatic, as if you’re stressing ο φίλος μου.

Why is it ο φίλος μου and not something like μου φίλος?

The possessive pronoun μου (my) is a weak, enclitic pronoun. In Greek it normally comes after the noun it modifies:

ο φίλος μου = my friend
το σπίτι μου = my house
η μητέρα μου = my mother

Putting μου before the noun (μου φίλος) is not standard modern Greek and sounds wrong in this context. So the normal pattern is:

article + noun + μου

Why do we need the article ο before φίλος? Could we just say φίλος μου?

In standard, careful Greek, you normally use the definite article with nouns referring to specific people, including with possessives:

ο φίλος μου = my (specific) friend
η αδερφή μου = my sister

You can hear φίλος μου without ο in speech, especially in emotional or informal contexts, but:

ο φίλος μου is the regular, neutral form.
In a full sentence like this, ο φίλος μου is what you’d expect in writing and in normal speech.

Why is δουλεύει used, and what tense is it?

Δουλεύει is the 3rd person singular, present tense of δουλεύω (to work):

δουλεύω = I work
δουλεύεις = you work
δουλεύει = he/she/it works

In this sentence:

όπου δουλεύει ο φίλος μου = where my friend works

The Greek present tense here covers both English “works” (simple present) and “is working” (present continuous), depending on context. For a general fact (he works there regularly), present tense δουλεύει is exactly right.

Why is it ο κεντρικός σταθμός and not τον κεντρικό σταθμό?

Ο κεντρικός σταθμός is in the nominative case because it is the subject of the verb είναι (is):

Ο κεντρικός σταθμός … είναι …
→ The central station … is …

If it were the object of a verb, you would see the accusative:

Βλέπω τον κεντρικό σταθμό. = I see the central station.

So:

Subject → ο κεντρικός σταθμός (nominative)
Object → τον κεντρικό σταθμό (accusative)

Is κτίριο the only correct spelling? I’ve seen κτήριο too.

Both κτίριο and κτήριο exist in modern Greek:

κτίριο is more common in everyday use and in much contemporary writing.
κτήριο is also accepted and appears in some dictionaries and official texts.

For you as a learner, using κτίριο is completely safe and standard. The meaning and grammar are identical in this sentence.