Learn Hardcore Greek: Δεν μου αρέσει όταν η διαφήμιση είναι πολύ δυνατή στο ραδιόφωνο. - I don’t like it when the advertisement is very loud on the radio.

Greek normally uses this impersonal structure with αρέσει to express likes and dislikes, instead of saying something like "εγώ αρέσω αυτό", which would be wrong in this meaning.

Why is μου (to me) in the genitive case here?

Clitic pronouns like μου, σου, του in Greek are often in the genitive when they show possession or an indirect object ("to me," "for me").

In μου αρέσει:

αρέσει is the verb
μου is the person to whom something is pleasing → "to me"

So μου is genitive because it functions as an indirect object:

μου αρέσει η διαφήμιση = "the advertisement pleases me" → "I like the advertisement."

You cannot replace this with με (accusative) or εγώ; μου is the only correct choice here.

Why does δεν come before μου αρέσει and not in another position?

The standard position of the negation δεν in Greek is immediately before the verb (or before the clitic pronouns attached to the verb phrase).

Here, the verb is αρέσει, with the clitic μου attached to it. The whole unit is μου αρέσει, so:

Δεν μου αρέσει = correct
Μου δεν αρέσει = wrong
Δεν αρέσει μου = wrong in normal speech

So: δεν + (clitic) + verb → Δεν μου αρέσει.

Why is there a definite article η before διαφήμιση? In English we just say "advertising" or "commercials."

Greek uses the definite article much more than English, including:

with nouns in a general or generic sense
with abstract nouns

In this sentence:

η διαφήμιση doesn’t mean only "the specific advertisement," but more broadly "advertising / commercials" as a general phenomenon.

So:

Δεν μου αρέσει όταν η διαφήμιση είναι πολύ δυνατή στο ραδιόφωνο. ≈ "I don’t like it when the advertising is very loud on the radio."

Using η sounds natural and is the default in Greek for this kind of generic statement.

Does η διαφήμιση mean "advertisement" or "commercials"? Why is it singular?

The noun η διαφήμιση can mean:

an advertisement (a specific ad)
advertising / commercials in general (an uncountable meaning, like English "advertising")

Here, with όταν η διαφήμιση είναι πολύ δυνατή στο ραδιόφωνο, the speaker is talking about radio advertising in general, not one single ad. Greek often uses a singular count noun with an article where English would use an uncountable or plural noun.

So even though it’s grammatically singular, the meaning is "when the commercials/ads are very loud on the radio."

Why is δυνατή feminine singular? What is it agreeing with?

δυνατή is an adjective that means "loud" or "strong." Adjectives in Greek must agree with the noun they describe in:

gender
number
case

The noun η διαφήμιση is:

feminine
singular
nominative

So the adjective must be:

feminine singular nominative → δυνατή

Hence:

η διαφήμιση είναι δυνατή = "the advertisement is loud."

If the noun were plural (e.g. οι διαφημίσεις), the adjective would change:

οι διαφημίσεις είναι δυνατές.

Why is it πολύ δυνατή and not πολύ δυνατά?

Two different structures:

πολύ δυνατή
- πολύ here is an adverb meaning "very."
- It modifies the adjective δυνατή.
- δυνατή agrees with η διαφήμιση.
- So: "very loud" (adverb + adjective).
πολύ δυνατά
- δυνατά is an adverb ("loudly").
- This would modify a verb: "to play very loudly," etc.

In the sentence, δυνατή is describing the advertising (a noun), not how something is being done, so the adjective is needed:

η διαφήμιση είναι πολύ δυνατή (the advertising is very loud), not
η διαφήμιση είναι πολύ δυνατά (ungrammatical; adverb cannot describe a noun).

Could the word order be Δεν μου αρέσει όταν στο ραδιόφωνο η διαφήμιση είναι πολύ δυνατή? Is that still correct?

Yes, that word order is grammatically correct:

Δεν μου αρέσει όταν στο ραδιόφωνο η διαφήμιση είναι πολύ δυνατή.

Greek word order is relatively flexible. The main differences are in emphasis and style, not grammar. Some common variants:

Δεν μου αρέσει όταν η διαφήμιση είναι πολύ δυνατή στο ραδιόφωνο.
Δεν μου αρέσει όταν η διαφήμιση στο ραδιόφωνο είναι πολύ δυνατή.
Δεν μου αρέσει όταν στο ραδιόφωνο η διαφήμιση είναι πολύ δυνατή.

They all mean the same thing; the original is perhaps the most neutral.

What exactly is στο in στο ραδιόφωνο?

στο is a contraction of the preposition σε + the neuter definite article το:

σε = "in / at / on"
το ραδιόφωνο = "the radio"
σε + το ραδιόφωνο → στο ραδιόφωνο = "on the radio"

This type of contraction is extremely common:

σε + τον → στον
σε + την → στη(ν)
σε + το → στο

So στο ραδιόφωνο literally means "at/on the radio."

Why is the verb είναι (is) in the present tense after όταν? Could another tense be used?

όταν here introduces a general condition or habitual situation: "when advertising is very loud on the radio (in general / whenever that happens)."

In Greek, for general truths and repeated situations, the present tense is used:

Δεν μου αρέσει όταν η διαφήμιση είναι πολύ δυνατή στο ραδιόφωνο.

If you were talking about a specific occasion in the past, you would change both verbs to past:

Δεν μου άρεσε όταν η διαφήμιση ήταν πολύ δυνατή στο ραδιόφωνο. = "I didn’t like it when the advertising was very loud on the radio (that time)."

So the present είναι matches the general, habitual meaning.

Can you drop the verb είναι and say «όταν η διαφήμιση πολύ δυνατή στο ραδιόφωνο»?

No, you cannot omit είναι here. Greek almost always requires the verb "to be" (είμαι) in such sentences:

η διαφήμιση είναι πολύ δυνατή = the advertising is very loud.

Leaving out είναι as in:

η διαφήμιση πολύ δυνατή is not grammatical in standard Modern Greek.

Unlike some languages that often drop "to be" in the present, Greek does not normally do that in this type of sentence.

Why is there sometimes δε instead of δεν, as in Δε μου αρέσει?

δεν is the full form of the negation word. Before consonants, especially in fast or informal speech, the final -ν is often dropped, giving δε:

Δεν μου αρέσει → Δε μου αρέσει

Both are correct; in writing, δεν is slightly more standard, but δε is very common and natural, especially in casual writing and speech. The meaning is exactly the same.