Learn Hardcore Greek: Ο καθηγητής μου κάνει παρατήρηση όταν μιλάω αγγλικά στην τάξη. - My professor makes a remark to me when I speak English in class.

Putting μου before the noun (e.g. μου καθηγητής) is wrong in standard Modern Greek.
You also don’t use the article before the possessive like in English (the my teacher); the article goes only before the noun:

✅ Ο φίλος μου = my friend
❌ ο μου φίλος
❌ μου φίλος

Does μου here mean “my” or “to me”? I know μου can mean both.

You’re right: μου can be:

Possessive: my
Indirect object: to me / for me

In Ο καθηγητής μου κάνει παρατήρηση, the μου is attached to the noun καθηγητής, so it’s possessive:

Ο καθηγητής μου = my teacher

The “to me” idea (tells me off) is understood from context and the verb phrase κάνει παρατήρηση. Greek often omits an obvious object pronoun.

If you wanted to make “to me” very explicit, you could say:

Ο καθηγητής μού κάνει παρατήρηση.
(Here μού is stressed and functions clearly as to me.)

Or even:

Ο καθηγητής μου μού κάνει παρατήρηση.
First μου = my, second μού = to me.

What does the expression κάνει παρατήρηση literally mean, and why is it used for “tells me off / scolds me”?

Literally:

κάνει = he/she does / makes
παρατήρηση = remark / comment / observation

So κάνει παρατήρηση literally is “makes a remark”.

Idiomatic meaning in everyday Greek, especially in a school/authority context, is:

κάνει (σε κάποιον) παρατήρηση = to tell someone off, to reprimand, to scold, to warn them about their behavior

So in this sentence:

Ο καθηγητής μου κάνει παρατήρηση
= My teacher tells me off / reprimands me.

A stronger, more obviously “scolding” verb is:

με μαλώνει = he/she scolds me

Why is there no article before παρατήρηση? Why not κάνει μια παρατήρηση?

Both are grammatically possible, but there’s a nuance:

κάνει παρατήρηση (no article)
– Generic / habitual action: he tells me off (in general, as a habit)
– That’s why it fits well with όταν (when(ever)).
κάνει μια παρατήρηση (with μια)
– Focus on a single, specific instance: he makes a (single) remark / tells me off once
– More like describing one event, not a repeated pattern.

In this sentence, the meaning is whenever I speak English in class, he (habitually) tells me off, so no article is the natural choice.

Why is it όταν and not αν here? What’s the difference in this sentence?

όταν = when(ever) (for time)
αν = if (for condition)

In:

όταν μιλάω αγγλικά στην τάξη
= when(ever) I speak English in class

you’re talking about a time situation that actually happens and is viewed as a repeated fact.

If you used αν:

Αν μιλάω αγγλικά στην τάξη…
it would sound more like a hypothetical “if I happen to speak English in class” (less about a known repeated pattern, more about a condition).

What’s the difference between μιλάω and μιλώ? Can I use μιλώ instead?

μιλάω and μιλώ are two forms of the same verb (to speak / to talk), 1st person singular present.

μιλάω – more common in everyday spoken Greek
μιλώ – slightly more formal or “bookish”, but also completely correct

You can say either:

όταν μιλάω αγγλικά στην τάξη
όταν μιλώ αγγλικά στην τάξη

Meaning is identical: when(ever) I speak English in class.

Why is αγγλικά plural and with no article? Shouldn’t “English” be singular like in English?

In Greek, many language names are neuter plural adjectives used as nouns:

αγγλικά (neuter plural) = English (language)
ελληνικά = Greek (language)
γαλλικά = French (language)

So:

μιλάω αγγλικά = I speak English.
Literally: I speak (things that are) English.

No article is needed when you talk about a language in this general way:

μιλάω αγγλικά (not τα αγγλικά here)
μιλάω ελληνικά

You do use the article when you refer to the language as a school subject or “the English language” as a thing:

Τα Αγγλικά είναι δύσκολα. = English (as a subject) is hard.
Η αγγλική γλώσσα = the English language.

Why is it στην τάξη and not σε την τάξη? What exactly does στην mean?

στην is the contraction of:

σε
- την → στην

σε = in / at / to (very general preposition)
την = feminine singular accusative article (the)

So:

σε την τάξη → στην τάξη = in the class / in the classroom

This contraction is standard and almost always used:

σε + τον → στον (στον δρόμο = in/on the road)
σε + το → στο (στο σπίτι = at home)

In this sentence, στην τάξη corresponds to English “in class” or “in the classroom”.

Does τάξη mean “classroom” or “lesson” or “grade”? What’s meant here?

τάξη can mean several related things, depending on context:

Classroom / class (as a place)
- Είμαι στην τάξη. = I’m in the classroom.
The group of students / the class
- Η τάξη γράφει τεστ. = The class is taking a test.
Grade / year in school
- Είμαι στην πέμπτη τάξη. = I’m in fifth grade.

In στην τάξη with μιλάω αγγλικά, the most natural reading is:

in class / in the classroom (during the lesson)

Could I change the word order and say: Όταν μιλάω αγγλικά στην τάξη, ο καθηγητής μου κάνει παρατήρηση?

Yes, that’s completely correct and very natural.

Greek word order is flexible. In your original:

Ο καθηγητής μου κάνει παρατήρηση όταν μιλάω αγγλικά στην τάξη.

the focus starts with “My teacher tells me off…” and then adds the condition.

With:

Όταν μιλάω αγγλικά στην τάξη, ο καθηγητής μου κάνει παρατήρηση.

you start with the condition “When I speak English in class…” and then state the result.

Both mean the same; it’s mostly about emphasis and flow.

How would I say “my (female) teacher” in this sentence?

For a female teacher, use η καθηγήτρια:

Η καθηγήτριά μου κάνει παρατήρηση όταν μιλάω αγγλικά στην τάξη.

Note the accents:

η καθηγήτρια (basic form, stress on -γή-)
With the enclitic μου, an extra accent appears: η καθηγήτριά μου

This double accent is a normal spelling rule when an enclitic follows a word stressed on the third syllable from the end.

How is αγγλικά pronounced? I’m not sure about the γγ combination.

αγγλικά is pronounced approximately:

[aŋgliˈka]

Details:

α = “a” as in father
γγ before a front vowel (like ι) is pronounced like ng in English
- g
  → αγγ- sounds like ang-g → aŋg
ι = “ee” sound
Stress is on the last syllable: αγ-γλι-κά

So you get something like ang-glee-KÁ.