Μου κάνει εντύπωση πόσο καλά μιλάς ελληνικά.

Literally, it is something like:

To me it makes an impression how well you speak Greek.

A more natural English rendering is:

I’m impressed by how well you speak Greek
or
It impresses me how well you speak Greek.

So the Greek structure is a bit less direct than the usual English one.

Μου means to me.

It is the weak pronoun form used for an indirect object. So in this sentence:

μου κάνει εντύπωση = it makes an impression on me

English often turns this into I am impressed or it impresses me, but Greek keeps the to me idea very naturally.

Because the verb is in the third person singular: κάνει = it makes / does.

The thing that is doing the making an impression is the whole idea:

πόσο καλά μιλάς ελληνικά = how well you speak Greek

So the sentence works like:

[How well you speak Greek] makes an impression on me.

Greek does not need to say an explicit it, so κάνει can appear without a stated subject before it.

Yes. κάνω εντύπωση is a very common expression meaning:

• to make an impression
• to impress
• to strike someone
• sometimes to seem surprising / noteworthy

So Μου κάνει εντύπωση... often means:

• I find it striking that...
• I’m impressed that...
• It surprises me that...

The exact English translation depends on context, but the Greek expression itself is very standard.

Because κάνει εντύπωση is an idiomatic expression, and in this expression εντύπωση usually appears without an article.

So Greek normally says:

μου κάνει εντύπωση = it impresses me / it strikes me

not usually:

μου κάνει την εντύπωση

The version without the article is the normal one in this meaning.

Πόσο here means how, in the sense of to what degree.

Καλά means well.

So:

πόσο καλά μιλάς ελληνικά = how well you speak Greek

This is very similar to English.
You are not asking a question here; it is an embedded clause after μου κάνει εντύπωση.

Μιλάς is the second person singular form of μιλάω / μιλώ = to speak.

So it means:

you speak

Because it is singular and informal, the sentence is addressed to one person in a familiar way.

If you were speaking formally to one person, or to more than one person, you would say:

Μου κάνει εντύπωση πόσο καλά μιλάτε ελληνικά.

After μιλάω (to speak), language names are very often used without an article:

• Μιλάω ελληνικά = I speak Greek
• Μιλάει αγγλικά = He/She speaks English

So ελληνικά here simply means Greek, as a language.

This is completely normal Greek usage.

Because in Greek, names of languages are normally written with a lower-case letter, not a capital letter.

So:

• ελληνικά = Greek
• αγγλικά = English
• γαλλικά = French

This is different from English, where language names are capitalized.

Yes. You can say:

Μου κάνει εντύπωση το πόσο καλά μιλάς ελληνικά.

This is also natural and can sound a little more explicit or slightly more emphatic.

Both are correct:

• Μου κάνει εντύπωση πόσο καλά μιλάς ελληνικά.
• Μου κάνει εντύπωση το πόσο καλά μιλάς ελληνικά.

The version without το is already very common and natural.

Yes, definitely.

It sounds like a warm, natural compliment. It tells the other person that their Greek level is impressive.

Depending on tone and context, it can mean:

• I’m impressed by how well you speak Greek
• It’s amazing how well you speak Greek
• I’m struck by how well you speak Greek

So if someone says this to a learner, it is usually very positive.