Learn Hardcore Greek: Έχω έναν καινούριο υπολογιστή, αλλά το παλιό τηλέφωνο ακόμα δουλεύει. - I have a new computer, but the old phone still works.

Because υπολογιστής is masculine and, after a verb like έχω, it goes in the accusative. The masculine accusative form of the indefinite article is έναν. Neuter would be ένα.

Masculine: ένας (nom), έναν (acc)
Neuter: ένα (nom/acc)

Why does υπολογιστής become υπολογιστή (without -ς)?

Greek marks case on nouns. Υπολογιστής (computer) is masculine in -της:

Nominative: υπολογιστής (subject form)
Accusative: υπολογιστή (object form; -ς drops)
Genitive: υπολογιστή

After έχω, you need the accusative: υπολογιστή.

Why is the adjective καινούριο and not καινούριος?

Adjectives agree with the noun in gender, number, and case. Here we have a masculine noun in the accusative, so the adjective takes the masculine accusative singular form: καινούριο. Note: for many -ος adjectives, masculine accusative ends in -ο, which looks the same as neuter nominative/accusative.

Could I say νέος instead of καινούριος?

Often yes, but there’s a nuance:

καινούριος/καινούργιος = brand new, newly bought/made (best for devices).
νέος = new/young; good for people or newer versions, but less specific about being brand-new. Here, καινούριος υπολογιστής is the most natural. Spelling variant καινούργιος is equally common.

Why is it το παλιό τηλέφωνο and not ο παλιός τηλέφωνο?

Because τηλέφωνο is neuter. The article and adjective must be neuter nominative singular: το παλιό τηλέφωνο. Masculine would be ο παλιός.

What case is το παλιό τηλέφωνο in?

Nominative. It’s the subject of the second clause (το παλιό τηλέφωνο = the one doing the action), so the article/adjective are nominative neuter: το παλιό.

Do I need the definite article in το παλιό τηλέφωνο?

Yes, if you mean a specific, known phone: το παλιό τηλέφωνο. Without an article it sounds generic; with ένα it means “an old phone” (unspecified): ένα παλιό τηλέφωνο.

Where would I put “my,” as in “my old phone”?

Greek uses a post-nominal clitic:

το παλιό μου τηλέφωνο = my old phone. For the first clause, if you want “my new computer” (implying possession):
Έχω τον καινούριο μου υπολογιστή (I have my new computer [with me/possess it]).
But to state you own a newly acquired one, you normally say Έχω έναν καινούριο υπολογιστή.

Is το here an article or the pronoun “it”?

In το παλιό τηλέφωνο, το is the definite article. As a pronoun (“it”), το would appear before a verb, e.g., Το έχω = I have it.

Why is there a comma before αλλά?

It’s standard to use a comma before αλλά when linking two independent clauses. You may omit it in very short sentences, but it’s good style here.

Can I use όμως or μα instead of αλλά?

αλλά = but (neutral default).
όμως = however; can appear medially or initially and is more “discourse-like”: Όμως, το παλιό τηλέφωνο...
μα = but; more informal/emphatic. All are acceptable with slight tone differences.

Can ακόμα move around, and is ακόμη different?

Placement: You can say Το παλιό τηλέφωνο ακόμα δουλεύει, Το παλιό τηλέφωνο δουλεύει ακόμα, or Ακόμα δουλεύει το παλιό τηλέφωνο. Word order shifts emphasis but all are natural.
ακόμα vs ακόμη: They’re interchangeable; ακόμη can feel a touch more formal/literary. Both mean “still/yet” depending on context.

How do I say “yet” in the negative?

Use ακόμα with negation:

Δεν δουλεύει ακόμα / Ακόμα δεν δουλεύει = It doesn’t work yet.
For “no longer,” use πια or πλέον: Δεν δουλεύει πια/πλέον.

Does δουλεύει mean “he works” too? Is it okay for machines?

Yes. δουλεύω means “work” for people and “function/operate” for devices. For a more technical tone with devices, you can use λειτουργεί.

Why is the subject pronoun “I” missing in the first clause?

Greek is a pro-drop language: verb endings show the subject. Έχω already means “I have.” You add Εγώ only for emphasis: Εγώ έχω...

Could I say κομπιούτερ instead of υπολογιστής?

Yes. κομπιούτερ is a common loanword, indeclinable and neuter:

Έχω ένα καινούριο κομπιούτερ.
Note the article switches to neuter ένα.

Can I put the adjective after the noun, like το τηλέφωνο το παλιό?

Yes. Greek has two attributive positions:

Pre-nominal (neutral): το παλιό τηλέφωνο.
Post-nominal (often emphatic/focused): το τηλέφωνο το παλιό.

Any common pitfalls with αλλά vs άλλα?

Yes.

αλλά = but (accent on the last syllable, double λ).
άλλα = other things (neuter plural of άλλος, accent on the first syllable).
They’re spelled/pronounced differently and mean different things.

Quick pronunciation tips for the key words?

Stress the syllables with accents:

Έχω (É-kho)
έναν (É-nan)
καινούριο (ke-NÚ-rio)
υπολογιστή (i-po-lo-yi-STÍ)
αλλά (a-LÁ)
το παλιό τηλέφωνο (to pa-LIÓ ti-LÉ-fo-no)
ακόμα (a-KÓ-ma)
δουλεύει (du-LÉ-vi)