Learn Hardcore Greek: Σχεδόν κάθε Κυριακή βλέπω ταινία με την αδερφή μου. - Almost every Sunday I watch a movie with my sister.

Κάθε Κυριακή σχεδόν βλέπω ταινία… – possible, but less neutral; it slightly emphasizes κάθε Κυριακή and puts σχεδόν after it.
Βλέπω ταινία σχεδόν κάθε Κυριακή… – also natural, with a bit more emphasis on ταινία first, then when.

The most neutral/simplest is the original: Σχεδόν κάθε Κυριακή…

Why is it κάθε Κυριακή without an article, instead of την Κυριακή?

Κάθε Κυριακή means every Sunday and expresses a habit / repeated action. With κάθε, you do not use an article:

Κάθε Κυριακή πηγαίνω στην εκκλησία. – Every Sunday I go to church.

Την Κυριακή, with the article, usually refers to this Sunday / that Sunday as a specific day:

Την Κυριακή βλέπω ταινία με την αδερφή μου. – On Sunday I’m watching a movie with my sister (this coming Sunday).

So:

κάθε Κυριακή → every Sunday (habit)
την Κυριακή → (on) Sunday (a particular one, from context)

Why is Κυριακή capitalized? Are all days of the week capitalized in Greek?

Yes. In modern Greek, the names of the days of the week are treated like proper names and are capitalized:

Δευτέρα – Monday
Τρίτη – Tuesday
Τετάρτη – Wednesday
Πέμπτη – Thursday
Παρασκευή – Friday
Σάββατο – Saturday
Κυριακή – Sunday

So Κυριακή is capitalized for the same reason Sunday is capitalized in English.

Why is there no article before ταινία? Why not μια ταινία or την ταινία?

Greek can omit the article in cases where English would use a or the, especially with general or habitual objects.

Comparisons:

Βλέπω ταινία.
– I watch movies / I watch a movie (general/habitual, the type of activity, not a specific film).
Βλέπω μια ταινία.
– I’m watching a / one movie (a single, unspecified film; often sounds more like a specific event happening now).
Βλέπω την ταινία.
– I’m watching the movie (a particular movie already known in the context).

In Σχεδόν κάθε Κυριακή βλέπω ταινία…, the focus is on the habit: watching movies as an activity on Sundays, so the bare ταινία works well and sounds natural.

Why is the verb βλέπω used here? Does it mean “I watch” or “I am watching”?

The present tense βλέπω covers both:

I watch (habitual / repeated)
I am watching (right now)

Greek doesn’t distinguish between simple present and present continuous the way English does.

So:

Σχεδόν κάθε Κυριακή βλέπω ταινία…
– means I watch a movie almost every Sunday (habitual).

If you wanted to emphasize that you are doing it now, you’d still say βλέπω but add context, e.g.:

Τώρα βλέπω ταινία. – I’m watching a movie now.

Can the word order change? For example, can I say Βλέπω ταινία σχεδόν κάθε Κυριακή με την αδερφή μου?

Yes, Greek word order is fairly flexible, and your version is grammatical:

Βλέπω ταινία σχεδόν κάθε Κυριακή με την αδερφή μου.

Possible orders (all correct, with slight changes in emphasis):

Σχεδόν κάθε Κυριακή βλέπω ταινία με την αδερφή μου.
Βλέπω ταινία σχεδόν κάθε Κυριακή με την αδερφή μου.
Με την αδερφή μου βλέπω ταινία σχεδόν κάθε Κυριακή.

The original order is neutral and common. Moving parts around tends to shift what is emphasized, but the basic meaning stays the same.

What form is την αδερφή? What gender and case is used, and why?

Αδερφή (sister) is a feminine noun.

In με την αδερφή μου:

την = feminine singular accusative definite article
αδερφή = noun in feminine singular accusative
με governs the accusative case, so its object must be in accusative.

So την αδερφή is “the sister” as the object of με (“with the sister”), grammatically:

με ποιον; – with whom?
με την αδερφή μου. – with my sister.

Why is it αδερφή here and not αδελφή? Is there a difference?

Both αδερφή and αδελφή are correct and mean sister.

αδερφή – more colloquial / everyday spelling, closer to modern pronunciation.
αδελφή – more formal / traditional spelling, closer to the ancient form.

In speech, they sound the same (most speakers say a-ther-FÍ). In everyday writing, αδερφή is very common. In more formal contexts, you may see αδελφή.

Why is the possessive μου after αδερφή instead of before, like in English?

In Greek, short possessive pronouns like μου, σου, του, της, μας, σας, τους usually come after the noun:

η αδερφή μου – my sister
ο αδερφός σου – your brother
το σπίτι μας – our house

So με την αδερφή μου literally is “with the sister my,” but in natural English it’s with my sister.

You don’t say με την μου αδερφή in Greek; that word order is wrong. The possessive is an enclitic attached after the noun phrase.

Could I write με τη αδερφή μου instead of με την αδερφή μου?

In standard spelling, before a word starting with a vowel (αδερφή), the feminine article is written with -ν:

την αδερφή
την ώρα
την Ελλάδα

So με την αδερφή μου is the normal, correct spelling.

You may sometimes see τη without ν in informal writing, but:

After με, before a vowel, the recommended form is την.
In speech, both τη(ν) are pronounced the same (/tin/ before a vowel).

So for correct written Greek, keep την here: με την αδερφή μου.

Is με always used for “with”? What about μαζί με?

Με is the basic preposition meaning with:

Πηγαίνω με την αδερφή μου. – I go with my sister.

Μαζί means together, and μαζί με is often used to emphasize togetherness:

Βλέπω ταινία μαζί με την αδερφή μου.
– I watch a movie together with my sister.

Both are correct:

με την αδερφή μου – with my sister
μαζί με την αδερφή μου – together with my sister (a bit more emphasis on doing it together).

How would this sentence change with different subjects (you, he, we, etc.)?

The verb βλέπω changes according to the subject. Keeping the rest the same:

Εγώ – Σχεδόν κάθε Κυριακή βλέπω ταινία με την αδερφή μου.
– I watch a movie almost every Sunday with my sister.
Εσύ – Σχεδόν κάθε Κυριακή βλέπεις ταινία με την αδερφή σου.
– You watch a movie almost every Sunday with your sister.
Αυτός / Αυτή / Αυτό – Σχεδόν κάθε Κυριακή βλέπει ταινία με την αδερφή του / της.
– He/she watches a movie almost every Sunday with his/her sister.
Εμείς – Σχεδόν κάθε Κυριακή βλέπουμε ταινία με την αδερφή μας.
– We watch a movie almost every Sunday with our sister.
Εσείς – Σχεδόν κάθε Κυριακή βλέπετε ταινία με την αδερφή σας.
– You (plural/polite) watch a movie almost every Sunday with your sister.
Αυτοί / Αυτές / Αυτά – Σχεδόν κάθε Κυριακή βλέπουν(ε) ταινία με την αδερφή τους.
– They watch a movie almost every Sunday with their sister.