Learn Hardcore Greek: Στη μέση της ταινίας μπαίνει πάντα μια δυνατή διαφήμιση στο ίντερνετ. - In the middle of the movie there is always a loud advertisement on the internet.

You may see both στη μέση and στην μέση in writing.
In modern Greek, the final -ν of την is usually dropped before many consonants, especially in everyday language, so στη μέση is very common.

Functionally, both mean “in the middle”; στη is just the contracted form of σε τη(ν).

Why is it της ταινίας? Why is ταινία in the genitive case here?

The phrase στη μέση της ταινίας literally means “in the middle of the film”.

Greek usually uses the genitive after μέση to show possession or “part of a whole”:

στη μέση του δρόμου – in the middle of the road
στη μέση της σελίδας – in the middle of the page
στη μέση της ταινίας – in the middle of the film

So ταινία → της ταινίας (genitive singular) because it’s “the middle of the film”.

What is the subject of the verb μπαίνει in this sentence? It feels like nothing is “entering”.

Grammatically, the subject of μπαίνει is μια δυνατή διαφήμιση.

Word order in Greek is flexible, so the sentence is:

Στη μέση της ταινίας (time/place phrase, put first for emphasis)
μπαίνει (verb)
πάντα (adverb – always)
μια δυνατή διαφήμιση (subject)
στο ίντερνετ (another place/sphere phrase)

So:

Μια δυνατή διαφήμιση μπαίνει πάντα στη μέση της ταινίας στο ίντερνετ.

is the more “English-like” order: “A strong commercial always comes in the middle of the film on the internet.”

Why use μπαίνει? Doesn’t μπαίνω mean “to enter / to go in”? How does that fit the idea of an advertisement?

Literally, μπαίνω = to enter, to go in.
In modern Greek, it’s also used figuratively for things that start, appear, or get inserted:

Μπαίνει διαφήμιση. – An ad comes on / an ad starts.
Μπαίνει μουσική. – Music starts playing.
Μπήκαν υπότιτλοι. – Subtitles were added.

So in this sentence:

μπαίνει πάντα μια δυνατή διαφήμιση
≈ “a strong commercial always comes on / is always inserted”

This is very natural Greek for an ad starting in the middle of something.

What does δυνατή διαφήμιση really mean? “Strong advertisement” sounds strange in English.

Literally, δυνατή = strong, powerful.

With διαφήμιση, δυνατή διαφήμιση often implies:

loud (sound is strong)
intense / in-your-face
very noticeable / impactful
sometimes effective / well-made

Depending on context, you might translate it as:

“a loud ad”
“a really strong / aggressive commercial”
“a very impactful ad”

Greek can use δυνατός/δυνατή/δυνατό more broadly than English strong, often meaning “impressive, intense, effective”.

Why is it μια δυνατή διαφήμιση and not η δυνατή διαφήμιση?

μια is the indefinite article = a / an.
η is the definite article = the.

μια διαφήμιση – a commercial (not a specific one already known)
η διαφήμιση – the commercial (a particular one you’ve already identified)

Here, we are talking in general: “a (some) loud ad always comes in the middle…”, not one specific ad that both speaker and listener already know.

So μια is the natural choice.

What is the difference between μια and μία? I see both spellings.

Both forms exist:

μια – the usual, unstressed indefinite article: a / an
μία – either:
- the number “one” (feminine: one woman, one movie), or
- an emphatic form of the article (“one single …”, “this one …”).

In everyday writing, many people don’t distinguish strictly and write μια for almost everything.

In your sentence, we clearly mean “a commercial”, not “one (single) commercial” with emphasis, so μια is perfect.
Μία δυνατή διαφήμιση would sound a bit more emphatic: “one strong ad”, if you wanted to stress the number or uniqueness.

Why is διαφήμιση feminine? Is there a rule for that?

Noun genders in Greek are mostly lexical (you simply have to learn them), but endings can give strong hints.

διαφήμιση ends in -ση, which very often marks feminine nouns:

η χρήση – use
η γνώση – knowledge
η άσκηση – exercise
η διαφήμιση – advertisement

So:

η διαφήμιση (nominative)
μια δυνατή διαφήμιση – a strong advertisement

The adjective δυνατή agrees with διαφήμιση in:

gender: feminine
number: singular
case: nominative (as the subject of μπαίνει)

Why is πάντα placed after μπαίνει? Could I move it?

Yes, πάντα (always) is fairly flexible in position. All of these are possible:

Στη μέση της ταινίας μπαίνει πάντα μια δυνατή διαφήμιση…
Στη μέση της ταινίας πάντα μπαίνει μια δυνατή διαφήμιση…
Στη μέση της ταινίας μπαίνει μια δυνατή διαφήμιση πάντα… (more colloquial / end-focus)

Differences are mostly about rhythm and emphasis, not grammar:

μπαίνει πάντα slightly groups “comes in + always” as a unit.
πάντα μπαίνει puts a touch more emphasis on “always (and not just sometimes)”.

The version in your sentence is very natural and common.

Why is it στο ίντερνετ? What does στο consist of, and why use an article with “internet”?

στο is a contraction of:

σε = in / at / on
το = the (neuter, singular)

So:

σε + το ίντερνετ → στο ίντερνετ

In Greek, even with borrowed words like ίντερνετ, speakers very often use the definite article:

το ίντερνετ – the internet
στο ίντερνετ – on the internet

Using an article with abstract or global things is normal in Greek (the internet, the cinema = το σινεμά, etc.).

Can ίσντερνετ also be said without the article, like just ίντερνετ?

Yes, you can sometimes see or hear στο ίντερνετ and sometimes στο ίντερνετ used more like a name without an article, but:

το ίντερνετ / στο ίντερνετ is still the more standard and common form.

In many fixed expressions and everyday speech, Greek tends to keep the article:

στο σχολείο, στο σινεμά, στη δουλειά, στο ίντερνετ

Why is the verb in the present tense μπαίνει, when we mean something that always happens, not just now?

In Greek, the present simple is used for:

actions happening now
habitual / repeated actions
general truths

Your sentence describes a habitual action:

Every time you watch the movie online, an ad always comes in the middle.

So the present tense μπαίνει correctly expresses this regular, repeated behavior, just like English present simple in “An ad always comes in the middle of the movie.”

Can I change the word order and say Μια δυνατή διαφήμιση μπαίνει πάντα στη μέση της ταινίας στο ίντερνετ? Does it sound okay?

Yes, that sentence is grammatically correct and understandable.

Μια δυνατή διαφήμιση μπαίνει πάντα στη μέση της ταινίας στο ίντερνετ.

This order:

starts with the subject μια δυνατή διαφήμιση, so it feels closer to English word order;
puts less initial emphasis on “in the middle of the movie” and more on “a strong ad”.

The original:

Στη μέση της ταινίας μπαίνει πάντα μια δυνατή διαφήμιση στο ίντερνετ.

pushes “in the middle of the movie” into the spotlight by placing it first. Both are natural; the difference is mostly where the emphasis falls.

Could I use a different verb instead of μπαίνει, like εμφανίζεται or παίζει? Would that change the nuance?

Yes, and each choice changes the nuance slightly:

εμφανίζεται πάντα μια δυνατή διαφήμιση
– “a strong ad always appears”
– more neutral / descriptive.
παίζει πάντα μια δυνατή διαφήμιση
– literally “a strong ad always plays”
– very common in spoken Greek about things on TV / radio / internet (shows, songs, commercials).
βάζουν πάντα μια δυνατή διαφήμιση
– “they always put a strong ad”
– focuses more on the people/platform that insert the ad.

μπαίνει is a nice in-between: casual, very natural, and doesn’t mention who is doing the inserting.