Learn Hardcore Greek: Στον φάκελο βάζω και μια κάρτα με τη διεύθυνσή μου, για να μου τον στείλουν αν χαθεί. - In the envelope I also put a card with my address so that they send it to me if it gets lost.

μια κάρτα: usually just “a card” (indefinite article, neutral tone)
μία κάρτα: can emphasize the number “one card” (exactly one), though in practice many writers don’t keep a strict distinction.

Pronunciation is often the same in everyday speech; the spelling μία is often used when you want to stress the idea of “one (single)”. Here, μια κάρτα fits well as a simple “a card”.

Why do we say με τη διεύθυνσή μου and not just διεύθυνσή μου without τη?

In Greek, when you have a noun with a possessive pronoun (like μου, σου, του), you normally still keep the definite article.

η διεύθυνση = the address
η διεύθυνσή μου = my address
με τη διεύθυνσή μου = with my address

So, the pattern is:
[prep] + [definite article] + [noun] + [possessive clitic]

You would not usually drop the article here. Saying just με διεύθυνσή μου is ungrammatical in standard modern Greek.

Why is the accent in διεύθυνσή μου on the last syllable instead of διεύθυνση?

The base word is διεύθυνση (accent on the -εύ- syllable).

When you add an enclitic pronoun like μου, Greek often adds a second accent on the last syllable to keep proper stress rules:

η διεύθυνση
η διεύθυνσή μου

So διεύθυνσή μου is just the normal accented form of “my address”. The extra accent mark appears because of the following enclitic μου.

In για να μου τον στείλουν, what does για να do? Is it like an infinitive?

Greek doesn’t use an infinitive the way English does. Instead, it often uses να (or για να) plus a finite verb in the subjunctive.

να στείλουν ≈ “to send” / “(that) they send”
για να στείλουν = “in order to send / so that they send”

So για να introduces a purpose clause:

βάζω κάρτα …, για να μου τον στείλουν
→ I put in a card … so that they can send it to me.

In meaning, για να + subjunctive corresponds closely to English “in order to + verb”.

Why is στείλουν used (subjunctive), and not στέλνουν (present indicative)?

After να / για να, Greek normally uses the subjunctive, not the indicative.

Present indicative: στέλνουν = they send / they are sending
Aorist subjunctive: στείλουν = (that) they send (as a whole event)

Για να μου τον στείλουν focuses on the single future event of sending the envelope back to you. The aorist subjunctive is the normal choice for a one-time, completed action in such purpose clauses.

In να μου τον στείλουν, what does μου mean? Why is it in the genitive?

Here μου is an indirect object pronoun meaning “to me” / “for me”.

Greek usually expresses indirect objects with a genitive clitic instead of a separate preposition:

μου τον στείλουν = (that) they send it to me
literally: “they-send me-GEN it-ACC”

So μου here does the job that English does with “to me”. It is genitive because that’s the standard case for these object pronouns (μου, σου, του, της, μας, σας, τους).

In να μου τον στείλουν, what does τον refer to exactly?

Τον is a masculine accusative singular direct object pronoun. In this sentence, it refers back to τον φάκελο (“the envelope”).

So:

Στον φάκελο = in the envelope
Later: να μου τον στείλουν = so that they send it (the envelope) to me

Greek drops the noun and replaces it with τον when it’s clear from context what “him/it” refers to.

Why is the order μου τον στείλουν and not τον μου στείλουν?

Greek has a fixed preferred order for clitic pronouns before the verb. When you have both a genitive and an accusative clitic, the genitive normally comes first:

Genitive (indirect object)
Accusative (direct object)
Verb

So the normal order is:

μου τον στείλουν, σου την έδωσα, μας το είπαν, etc.

τον μου στείλουν is ungrammatical in standard modern Greek.

In να μου τον στείλουν, who are “they”? Why is it 3rd person plural?

Greek often uses the third person plural with no explicit subject to mean “people in general / whoever is responsible”, similar to English “they” or “you” in general statements.

Here στείλουν in 3rd person plural can be understood as:

“so that they (whoever finds it / the post office people / whoever) send it to me”

It’s an indefinite “they”, not a specific group already mentioned.

In αν χαθεί, what form is χαθεί and why is it used instead of something like αν χάνεται?

Χαθεί is the aorist passive subjunctive of the verb χάνομαι (“to get lost”).

Active: χάνω = I lose (something)
Passive: χάνομαι = I get lost / I am lost
Aorist passive subjunctive (3rd sg): (να) χαθεί

With αν (“if”), modern Greek typically uses the subjunctive, especially for future or uncertain events:

αν χαθεί = if it gets lost / if it should get lost

Using αν χάνεται would sound like a repeated or habitual action (“if it is habitually getting lost”), which is not what is meant here.

Could you say αν θα χαθεί instead of αν χαθεί?

In standard modern Greek, after αν (if), you almost always use subjunctive without θα:

Correct: αν χαθεί, αν έρθει, αν φύγουν
Generally avoided in standard speech: αν θα χαθεί

Some dialects or informal styles may occasionally use αν θα + verb, but in good standard Greek you should stick to:

αν + subjunctive → αν χαθεί = if it gets lost.

The word μου appears twice: τη διεύθυνσή μου and να μου τον στείλουν. Are these the same kind of μου?

They are the same form (genitive clitic μου), but they play different roles:

τη διεύθυνσή μου:
- μου = possessive: my address
να μου τον στείλουν:
- μου = indirect object: to me

Grammatically, Greek uses the same clitic set (μου/σου/του…) both for possession (my/your/his) and for indirect objects (to me/to you/to him), and context tells you which is which.