Learn Hardcore Greek: Αυτή η διαφήμιση στο ίντερνετ δεν είναι καθόλου ενδιαφέρουσα. - This advertisement on the internet is not interesting at all.

In Modern Greek, when you use a demonstrative like «αυτός / αυτή / αυτό» (this), you normally still keep the definite article «ο / η / το» in front of the noun.

So:

Αυτή η διαφήμιση = this advertisement
Η διαφήμιση = the advertisement
Αυτή alone usually works as “this one” and doesn’t directly precede a noun:
- Αυτή είναι καλή. = This one is good.

Using «Αυτή διαφήμιση» without «η» is ungrammatical in standard Modern Greek. The pattern is basically:

[demonstrative] + [article] + [noun]
αυτή η διαφήμιση, αυτός ο άνθρωπος, αυτό το παιδί

Why is «Αυτή» feminine? How does it agree with «διαφήμιση»?

Greek has grammatical gender, and adjectives, articles, and demonstratives must agree with the noun in:

gender (masculine, feminine, neuter)
number (singular, plural)
case (nominative, genitive, etc.)

Here:

διαφήμιση is a feminine noun (ending -η / -ση is very often feminine)
It is singular and in the nominative case (subject of the sentence)

So everything that goes with it also becomes feminine, singular, nominative:

demonstrative: αυτή (fem. nom. sg.)
article: η (fem. nom. sg.)
adjective: ενδιαφέρουσα (fem. nom. sg.)

That’s why we have: Αυτή η διαφήμιση … δεν είναι καθόλου ενδιαφέρουσα.

What is the structure and meaning of «στο ίντερνετ»? Why not just «σε ίντερνετ»?

«Στο» is a contraction of «σε» + «το»:

σε = in / at / on
το = the (neuter nominative/accusative singular article)

So:

σε + το = στο → στο ίντερνετ = on the internet

Greek almost always uses an article with countable nouns and many mass nouns, even in places where English might drop it. With place-like nouns, you typically say:

στο σχολείο = at school
στο σπίτι = at home
στο ίντερνετ = on the internet

Saying «σε ίντερνετ» without the article would sound incomplete or unnatural in standard Greek.

Why is it «στο ίντερνετ» and not «στον ίντερνετ»?

The choice is about gender:

στον = σε + τον (masculine: to the / at the for masculine nouns)
στο = σε + το (neuter: to the / at the for neuter nouns)

«Ίντερνετ» is treated as a neuter noun in Greek, and it’s indeclinable (it doesn’t change form in different cases), so you use το ίντερνετ, στο ίντερνετ, etc.

Therefore, the correct form is «στο ίντερνετ», not «στον ίντερνετ».

What exactly does «καθόλου» mean here?

In negative sentences, «καθόλου» means “at all” and works as an intensifier of the negation:

δεν είναι καθόλου ενδιαφέρουσα
= it is not at all interesting / it’s not interesting at all

So the structure:

δεν (negation) + είναι (is) + καθόλου (at all) + ενδιαφέρουσα (interesting)

makes the negative stronger than just «δεν είναι ενδιαφέρουσα» (it’s not interesting).

Can «καθόλου» appear in a positive sentence, or only with «δεν»?

It appears mostly with negatives, but not only.

With negation (most common):
- Δεν είναι καθόλου δύσκολο.
  It’s not difficult at all.
In questions, often implying a negative expectation:
- Σου άρεσε καθόλου;
  Did you like it at all?
In some positive, more colloquial uses, it can mean “quite / very”:
- Είναι καθόλου καλό; (colloquial, regional; not everyone uses this)
  can be understood as “Is it any good?” or even “Is it quite good?” depending on context.

For learners, the safest and most standard pattern to remember is:

δεν + verb + καθόλου + adjective/adverb → not at all …

Why is «ενδιαφέρουσα» ending in -ουσα? Is it an adjective or a participle?

«Ενδιαφέρουσα» is the feminine singular form of the adjective «ενδιαφέρων, -ουσα, -ον» (interesting).

Historically, it comes from a present participle of the verb ενδιαφέρω (to interest), but in Modern Greek it is used as a normal adjective:

ενδιαφέρων (masc.)
ενδιαφέρουσα (fem.)
ενδιαφέρον (neut.)

It agrees with «διαφήμιση» (feminine singular), so we get:

μια ενδιαφέρουσα διαφήμιση = an interesting advertisement
η διαφήμιση δεν είναι ενδιαφέρουσα = the advertisement is not interesting

Could the word order be «δεν είναι ενδιαφέρουσα καθόλου» instead of «δεν είναι καθόλου ενδιαφέρουσα»?

Yes, both are grammatically correct:

Δεν είναι καθόλου ενδιαφέρουσα.
→ very natural, neutral order
Δεν είναι ενδιαφέρουσα καθόλου.
→ also correct; can sound a bit more emphatic, like stressing not interesting at all at the end

In practice, «δεν είναι καθόλου ενδιαφέρουσα» is probably the more common, idiomatic order in everyday speech.

Why do we use «δεν» instead of something like “no” or “not” attached to the verb?

In Greek, negation for verbs is done with a separate particle:

δεν (before most verbs)
μη(ν) (in imperatives, some subjunctives, certain structures)

Here we use δεν:

είναι = is
δεν είναι = is not

Unlike English, Greek doesn’t add -n’t onto the verb. The negative word always precedes the verb:

δεν θέλω = I don’t want
δεν καταλαβαίνω = I don’t understand
δεν είναι = it is not

Could I say «Αυτή η ίντερνετ διαφήμιση» instead of «Αυτή η διαφήμιση στο ίντερνετ»?

«Ίντερνετ» is normally used as a separate noun (the internet), not as an adjective in front of other nouns.

So:

Αυτή η διαφήμιση στο ίντερνετ = this advertisement on the internet (natural)
Αυτή η ίντερνετ διαφήμιση sounds strange in Greek.

If you want something shorter and more idiomatic, you might say:

Αυτή η διαδικτυακή διαφήμιση
(this online / internet advertisement)

Here «διαδικτυακή» is a proper adjective meaning online / internet-based.

Is there a difference between «διαφήμιση» and «αγγελία»? Could I use «αγγελία» here?

They are related but not identical:

διαφήμιση = advertisement, commercial (broader term, usually promotional)
αγγελία = an ad/notice, often a classified (job ad, personal ad, small ad in a newspaper, etc.)

In the context of “on the internet”, «διαφήμιση» is the natural word for online ads / banner ads / commercials.

You could say «αυτή η αγγελία στο ίντερνετ», but that would usually refer more specifically to something like a small online classified ad (job posting, apartment listing, etc.), not just any online ad.

Can I move «αυτή» to the end and say «Η διαφήμιση στο ίντερνετ αυτή δεν είναι καθόλου ενδιαφέρουσα»?

You can move the demonstrative after the noun, but it normally comes right after the noun phrase, not all the way after the prepositional phrase:

Most natural patterns:

Αυτή η διαφήμιση στο ίντερνετ δεν είναι καθόλου ενδιαφέρουσα.
Η διαφήμιση αυτή στο ίντερνετ δεν είναι καθόλου ενδιαφέρουσα.

Both mean this advertisement on the internet is not at all interesting.

Your version:

Η διαφήμιση στο ίντερνετ αυτή δεν είναι καθόλου ενδιαφέρουσα.

is understandable, but sounds awkward and less natural. The usual rule is:

Demonstrative either before the article (Αυτή η διαφήμιση)
or right after the noun (Η διαφήμιση αυτή), not split apart by other phrases.

Why is only the first word «Αυτή» capitalized, and not «ίντερνετ»? Isn’t “internet” a proper noun?

In Greek, «ίντερνετ» is usually written with a lowercase initial, and often treated like a common noun. It’s a loanword, and usage has settled on lowercase in most contexts:

το ίντερνετ = the internet

You capitalize:

the first word of a sentence: Αυτή…
proper names: Αθήνα, Γιάννης, Ευρώπη etc.

Because Greek treats «ίντερνετ» more like a common technological term than a formal proper name, it normally appears in lowercase, just like τηλεόραση (television) or ραδιόφωνο (radio).