Learn Hardcore Greek: Συμφωνώ με τη φίλη μου, αλλά ο φίλος μου διαφωνεί. - I agree with my friend female, but my friend male disagrees.

Greek is pro‑drop: the verb ending in συμφωνώ already shows the subject is “I.” You add εγώ only for emphasis or contrast: Εγώ συμφωνώ, αλλά ο φίλος μου διαφωνεί.

What cases are used in τη φίλη and ο φίλος?

τη φίλη is feminine accusative singular because the preposition με “with” always takes the accusative.
ο φίλος is masculine nominative singular because it is the subject of διαφωνεί.

Why is it με after συμφωνώ? Is there a difference between με and μαζί (με)?

Greek says “agree with”: συμφωνώ με + accusative (e.g., συμφωνώ με τη φίλη μου, με σένα). You’ll also hear the very common variant with μαζί + genitive clitic: συμφωνώ μαζί σου.

με = preposition “with,” used with strong pronouns: με μένα/εμένα, με σένα.
μαζί = “together,” takes weak genitive clitics: μαζί μου/σου/του.
μαζί με often precedes a noun: μαζί με τη Μαρία.
Don’t say συμφωνώ τη φίλη μου (you need the preposition).

Why is it τη φίλη and not την φίλη?

The final -ν of την can drop before many consonants. Keep -ν before vowels and these consonants/clusters: κ, π, τ, ξ, ψ, μπ, ντ, γκ, τσ, τζ. Since φ isn’t in that set, τη φίλη is standard. Writing την φίλη is also common and not considered wrong in careful/formal style.

Why ο φίλος μου but τη φίλη μου? Does μου affect the article?

No. The article agrees with the noun’s gender and case:

Object of με (feminine accusative): τη φίλη μου.
Subject (masculine nominative): ο φίλος μου.
The possessive μου is an unstressed genitive clitic that doesn’t change for gender or number: ο φίλος μου, η φίλη μου, οι φίλοι μου, οι φίλες μου.

Why does μου come after the noun, and do I need the article?

Greek usually uses the definite article with possessives, and the possessive follows the noun:

η φίλη μου, ο φίλος μου = “my friend.”
Without the article it’s more indefinite: φίλος μου ≈ “a friend of mine.”

Can μου come before the noun?

Not by itself. For emphasis you use the adjective δικός/δική/δικό:

η δική μου φίλη = my friend (as opposed to someone else’s).
Plain η μου φίλη is archaic/poetic.

Could/should we add who he disagrees with in the second clause?

You can if you want to be explicit:

ο φίλος μου διαφωνεί με μένα (με εμένα/με μένα)
ο φίλος μου διαφωνεί μαζί μου
It’s omitted in the original because the context already makes it clear.

Why is there a comma before αλλά?

αλλά links two independent clauses, and Greek normally uses a comma before it: ..., αλλά .... Synonyms: μα (informal) and όμως (“however,” which can move: ..., όμως ... or Όμως, ...).

Can the word order change?

Yes, for emphasis:

Συμφωνώ με τη φίλη μου, αλλά διαφωνεί ο φίλος μου (focus on the contrast).
Εγώ συμφωνώ με τη φίλη μου, αλλά ο φίλος μου διαφωνεί (emphatic subject).

How would I say this in the past?

Aorist (single, completed action): Συμφώνησα με τη φίλη μου, αλλά ο φίλος μου διαφώνησε.
Imperfect (ongoing/habitual): Συμφωνούσα με τη φίλη μου, αλλά ο φίλος μου διαφωνούσε.

What are the present-tense forms of συμφωνώ and διαφωνώ?

συμφωνώ: συμφωνώ, συμφωνείς, συμφωνεί, συμφωνούμε, συμφωνείτε, συμφωνούν(ε)
διαφωνώ: διαφωνώ, διαφωνείς, διαφωνεί, διαφωνούμε, διαφωνείτε, διαφωνούν(ε)

Do ο φίλος μου / η φίλη μου ever mean “boyfriend/girlfriend”?

Usually they mean “male/female friend.” For romantic partners Greek more often uses το αγόρι μου (boyfriend) and η κοπέλα μου (girlfriend). Context can occasionally make ο φίλος μου mean “boyfriend,” but that isn’t the default.