Learn Hardcore Greek: Η διπλανή τάξη κάνει μάθημα ελληνικών. - The neighboring class is having a Greek lesson.

It is used because τάξη (class) is a feminine noun in Greek, and it’s the subject of the sentence.
Nouns and their articles must match in gender, number, and case, so we say:

η τάξη = the class
(not ο τάξη or το τάξη, because those are masculine and neuter articles)

Why is it διπλανή and not something like δίπλα?

Both relate to the idea of “next to / beside,” but they are different parts of speech:

δίπλα = an adverb meaning “next to, beside”
- e.g. Το σχολείο είναι δίπλα στο σπίτι μου.
  The school is next to my house.
διπλανός / διπλανή / διπλανό = an adjective meaning “next-door / adjacent / neighboring”
It changes form to agree with the noun:
- διπλανός φίλος (masc.)
- διπλανή τάξη (fem.)
- διπλανό δωμάτιο (neuter)

In the sentence, we need an adjective to describe τάξη (class), so we use the feminine form διπλανή to agree with τάξη.

Why does διπλανή end in -ή?

Διπλανή is the feminine singular nominative form of the adjective διπλανός (next-door, neighboring).

Greek adjectives change their endings to match the noun’s:

Masculine: διπλανός (π.χ. διπλανός μαθητής – the student next to me)
Feminine: διπλανή (π.χ. διπλανή τάξη – the neighboring class)
Neuter: διπλανό (π.χ. διπλανό δωμάτιο – the room next door)

Since τάξη is feminine and is the subject of the sentence, the adjective also appears in feminine nominative singular, so we get διπλανή τάξη.

What exactly does τάξη mean here?

Τάξη can mean a few related things in Greek, depending on context:

Classroom (physical room)
- e.g. Οι μαθητές είναι μέσα στην τάξη.
  The students are in the classroom.
Class / group of students
- e.g. Η τάξη γράφει τεστ.
  The class is writing a test.
School grade / year
- e.g. Πάω στην τρίτη τάξη.
  I am in third grade.

In Η διπλανή τάξη κάνει μάθημα ελληνικών, it most naturally means the neighboring class (group of students in the next room), but context could also suggest “the classroom next door.”

Why do we say κάνει μάθημα instead of something like “studies Greek” with one verb?

Greek often uses the verb κάνω (to do, to make) with a noun to express what English might say with a single verb.

Κάνω μάθημα literally is “I do a lesson / I have a lesson / I’m in class”.

Some common patterns:

κάνω μάθημα = to have a lesson / to be having a class
κάνω γυμναστική = to do exercise / to work out
κάνω δουλειά = to do work

So η διπλανή τάξη κάνει μάθημα ελληνικών corresponds to something like:

“The class next door is having a Greek lesson.” or
“The next-door class is in Greek class.”

Why is the present tense κάνει translated as “is having” and not just “has”?

Modern Greek has one present tense that covers both:

English simple present: The class has a Greek lesson.
English present continuous: The class is having a Greek lesson.

Κάνει can mean:

it does / it makes / it has (repeated, usual action)
it is doing / is having (right now)

Context usually tells you whether it’s a habit or something happening now.
In this sentence, everyday usage favors an English continuous translation:
“The class next door is having a Greek lesson.”

Why is there no word like “is” (είναι) in the sentence?

English often needs the verb “to be” (is/are), but Greek doesn’t always mirror that structure.

Here, the main verb is κάνει (does / is doing / is having).
The sentence already has a full verb, so we don’t add είναι:

Correct: Η διπλανή τάξη κάνει μάθημα ελληνικών.
Wrong: Η διπλανή τάξη είναι κάνει μάθημα ελληνικών.

You would only use είναι if the sentence needed “is” as the main verb, for example:

Η διπλανή τάξη είναι θορυβώδης.
The class next door is noisy.

What does μάθημα mean, and why is it used here?

Μάθημα is a neuter noun meaning:

lesson, class session
and sometimes more generally course / subject (depending on context).

Examples:

Έχουμε μάθημα μαθηματικών.
We have a math lesson / math class.
Το μάθημα αρχίζει στις οκτώ.
The lesson starts at eight.

In κάνει μάθημα ελληνικών, it refers to a class session / lesson.
So the phrase means “is having a Greek lesson / is in Greek class.”

Why is ελληνικών in that form, and what case is it?

Ελληνικών is the genitive plural form, from:

τα ελληνικά = the Greek language (literally: “the Greek [things/words]”)
Genitive plural: των ελληνικών

In combinations like:

μάθημα ελληνικών
βιβλίο ελληνικών
καθηγητής ελληνικών

the genitive case indicates “of Greek” → a lesson of Greek, a Greek book (for learning Greek), teacher of Greek, etc.

So:

μάθημα ελληνικών = a lesson of Greek, i.e. a Greek lesson / Greek class.

This is a very common pattern in Greek:
noun + genitive plural of a language to express “[subject]-class / lesson / teacher / book”:

μάθημα αγγλικών (English class)
δάσκαλος γαλλικών (French teacher)
βιβλίο γερμανικών (German book / German textbook)

Could we say μάθημα ελληνικά instead of μάθημα ελληνικών?

No, not in standard Greek.

Ελληνικά (neuter plural nominative/accusative) is used on its own for “Greek (the language)”:
- Μιλάω ελληνικά. = I speak Greek.
- Μαθαίνω ελληνικά. = I’m learning Greek.

But when you put it after another noun to mean “lesson of Greek / book of Greek / teacher of Greek”, you use the genitive plural:

μάθημα ελληνικών
βιβλίο ελληνικών
καθηγητής ελληνικών

So:

❌ μάθημα ελληνικά – ungrammatical
✅ μάθημα ελληνικών – correct

Is the word order “διπλανή τάξη” fixed, or can the adjective go after the noun?

The usual and most neutral position for adjectives in Greek is before the noun:

η διπλανή τάξη = the neighboring class
το μεγάλο σπίτι = the big house
ο καλός φίλος = the good friend

Adjectives can sometimes appear after the noun, but that usually has:

a more emphatic or stylistic effect, or
is used in specific fixed expressions.

For this everyday sentence, the natural order is:

Η διπλανή τάξη κάνει μάθημα ελληνικών.

Putting it after (η τάξη διπλανή) would sound odd or poetic/marked, not normal conversational Greek.

How would the sentence change if we talked about more than one neighboring class?

You need to make everything that refers to the subject plural:

Η διπλανή τάξη κάνει μάθημα ελληνικών.
→ singular: The neighboring class is having a Greek lesson.

Becomes:

Οι διπλανές τάξεις κάνουν μάθημα ελληνικών.
→ plural: The neighboring classes are having a Greek lesson.

Changes:

Η → Οι (feminine plural nominative article)
διπλανή → διπλανές (feminine plural nominative adjective)
τάξη → τάξεις (feminine plural nominative noun)
κάνει → κάνουν (3rd person plural of κάνω)

How is this sentence pronounced, and where is the stress?

Syllable division and stress (stressed syllables in bold):

Η – /i/ (no stress, very short)
δι-πλα-νή – /ði-pla-ni/ (stress on the last syllable)
τά-ξη – /ta-ksi/ (stress on the first syllable)
κά-νει – /ka-ni/ (stress on the first syllable)
μά-θη-μα – /ma-thi-ma/ (stress on the first syllable)
ελ-λη-νι-κών – /e-li-ni-kon/ (stress on the last syllable; the double λ is just like a normal single [l] in modern Greek)

So roughly:

i ði-pla-NI TA-ksi KA-ni MA-thi-ma e-li-ni-KON

Remember:

The written accent (´) in Greek always marks the stressed syllable.
Η η is pronounced like English “ee” in see.