Learn Hardcore Greek: Η φίλη μου είναι χαρούμενη σήμερα, αλλά ήταν λυπημένη την Κυριακή. - My friend female is happy today, but she was sad on Sunday.

So the pattern is article + noun + possessive.
It sounds wrong or very incomplete to drop the article and say just φίλη μου in this kind of sentence.

Why is it φίλη here and not φίλος?

Greek nouns have grammatical gender.

φίλη is the feminine form meaning (female) friend.
φίλος is the masculine form meaning (male) friend.

Because the sentence is talking about a female friend, it uses the feminine:
Η φίλη μου είναι χαρούμενη…

Why is μου after the noun (φίλη μου) and not before it, like in English my friend?

The unstressed possessive pronouns (μου, σου, του, της, μας, σας, τους) normally come after the noun:

η φίλη μου = my friend
ο αδελφός σου = your brother
το βιβλίο της = her book

You can put a stressed form before the noun (η δική μου φίλη) to emphasize my (as opposed to someone else’s friend), but the neutral, everyday order is:
article + noun + unstressed possessive → η φίλη μου.

What is the difference between είναι and ήταν?

Both come from the verb είμαι (to be).

είναι = is / are (present tense, 3rd person singular or plural, depending on context)
- η φίλη μου είναι χαρούμενη = my friend is happy
ήταν = was / were (past tense, 3rd person singular or plural)
- …αλλά ήταν λυπημένη = but was sad

So the sentence contrasts present and past:
είναι (now) vs ήταν (before).

Why do χαρούμενη and λυπημένη both end in -η?

χαρούμενη and λυπημένη are feminine singular adjectives in the nominative case, agreeing with η φίλη.

Basic agreement:

Feminine: η φίλη → χαρούμενη, λυπημένη
Masculine: ο φίλος → χαρούμενος, λυπημένος
Neuter: το παιδί → χαρούμενο, λυπημένο

So we say:

Η φίλη μου είναι χαρούμενη / λυπημένη.
Ο φίλος μου είναι χαρούμενος / λυπημένος.

Can I change the word order? For example, say Η φίλη μου χαρούμενη είναι σήμερα?

Some changes are natural, others sound strange:

✅ Natural options:

Η φίλη μου είναι χαρούμενη σήμερα. (original)
Η φίλη μου σήμερα είναι χαρούμενη.
Σήμερα η φίλη μου είναι χαρούμενη.
Η φίλη μου είναι σήμερα χαρούμενη.

❌ Odd / wrong:

Η φίλη μου χαρούμενη είναι σήμερα. (sounds unnatural)

In general:

The verb είναι tends to stay close to the subject.
Adverbs like σήμερα are fairly flexible.
Greek allows some movement for emphasis, but not any random order.

Why is there no she in the second part (before ήταν)? Where is the subject?

Greek often omits the subject pronoun (like εγώ, εσύ, αυτός / αυτή…) because the verb form usually shows who is doing the action.

In …αλλά ήταν λυπημένη την Κυριακή, the subject is understood:

It is still η φίλη μου from the first clause.
You could say αυτή ήταν λυπημένη, but it’s not necessary and would sound more emphatic, like she was the one who was sad.

So Greek prefers:

Η φίλη μου είναι χαρούμενη σήμερα, αλλά (αυτή) ήταν λυπημένη την Κυριακή.

Why does σήμερα have no article, but την Κυριακή has την?

Two different patterns:

σήμερα (today) is an adverb. It works by itself, with no article:
- σήμερα, αύριο, χθες: no article.
την Κυριακή uses a noun (Sunday) in the accusative with the definite article, and in this structure it means on Sunday:
- την Δευτέρα = on Monday
- την Τρίτη = on Tuesday
- την Κυριακή = on Sunday

So:

σήμερα = today
την Κυριακή = on Sunday

Why is it την Κυριακή and not η Κυριακή here?

The article changes form according to case:

η Κυριακή → nominative (subject form)
την Κυριακή → accusative

For days of the week, Greek often uses article + accusative to say on [day]:

Την Κυριακή ήταν λυπημένη. = She was sad on Sunday.
Τη Δευτέρα δουλεύω. = I work on Monday.

If you say Η Κυριακή είναι η αγαπημένη μου μέρα, Κυριακή is the subject, so it stays in the nominative η Κυριακή.

Why is Κυριακή capitalized? Are all days of the week feminine?

In Modern Greek:

Days of the week are capitalized:
- Δευτέρα, Τρίτη, Τετάρτη, Πέμπτη, Παρασκευή, Σάββατο, Κυριακή
Not all are feminine, but Κυριακή is feminine, so it uses:
- article η / την Κυριακή

Others:

η Δευτέρα, η Τρίτη, η Τετάρτη, η Πέμπτη, η Παρασκευή (feminine)
το Σάββατο (neuter)
η Κυριακή (feminine)

Why is there a comma before αλλά?

αλλά means but and is linking two full clauses:

Η φίλη μου είναι χαρούμενη σήμερα
(αλλά) ήταν λυπημένη την Κυριακή

In Greek, as in English, when αλλά joins two complete sentences, a comma is normally placed before it:

…, αλλά …

If αλλά only links two small words or short phrases, you may sometimes see it without a comma, but between two clauses, the comma is standard.

How would I say the negative: My friend is not happy today, and she was not happy on Sunday?

You use δεν before the verb είναι / ήταν:

Η φίλη μου δεν είναι χαρούμενη σήμερα και δεν ήταν χαρούμενη την Κυριακή.

Pattern:

δεν + είναι = is not
δεν + ήταν = was not

Note that δεν comes right before the verb.