Learn Hardcore Greek: Μέχρι τις δέκα θα έχω ετοιμάσει τη βαλίτσα μου. - By ten o’clock I will have packed my suitcase.

So θα έχω ετοιμάσει corresponds to “I will have prepared / I will have packed” in English.
It expresses that the action will be completed before a specific future time (μέχρι τις δέκα = by ten o’clock).

What is the difference between θα έχω ετοιμάσει and θα ετοιμάσω?

Both are future, but they don’t say exactly the same thing.

Θα ετοιμάσω τη βαλίτσα μου
= I will pack my suitcase (at some point in the future).
Focus: the action will happen in the future, but not necessarily finished by any particular deadline.
Μέχρι τις δέκα θα έχω ετοιμάσει τη βαλίτσα μου
= By ten o’clock I will have packed my suitcase.
Focus: by the future time 10 o’clock, the action will already be completed.

You normally use θα έχω ετοιμάσει when you mention a future deadline (μέχρι…, ως…, πριν από…).

Why is it μέχρι τις δέκα and not something like μέχρι δέκα or μέχρι δέκα η ώρα?

Greek usually talks about clock time using a feminine plural noun ώρες (hours), often omitted but understood:

στις (τις) δέκα (η ώρα)
literally: at the ten (hours / o’clock)

So:

μέχρι τις δέκα
= by ten o’clock (literally: up to the ten (hours))

You could also say:

μέχρι τις δέκα η ώρα – more explicit, slightly heavier
μέχρι δέκα – possible in speech, but less natural; natives almost always keep τις in this expression.

In time expressions with the clock, the article τις is very standard:
στις τρεις, στις πέντε, μέχρι τις οχτώ, etc.

Why is the article τις feminine plural here? What noun is it agreeing with?

Τις is the accusative feminine plural definite article.
It agrees with an implied noun:

τις (ώρες) = the (hours)

In everyday Greek, people usually omit the word ώρες, but the article stays:

μέχρι τις (ώρες) δέκα → μέχρι τις δέκα

So:

τις agrees with ώρες, even though ώρες is not spoken.

This is why it’s not masculine or singular; it’s matching the (understood) feminine plural ώρες.

Could the sentence also be Θα έχω ετοιμάσει τη βαλίτσα μου μέχρι τις δέκα? Is that still correct?

Yes, that word order is perfectly correct:

Μέχρι τις δέκα θα έχω ετοιμάσει τη βαλίτσα μου.
Θα έχω ετοιμάσει τη βαλίτσα μου μέχρι τις δέκα.

Both mean the same thing. The difference is just emphasis:

Starting with Μέχρι τις δέκα slightly emphasizes the deadline.
Starting with Θα έχω ετοιμάσει… sounds a bit more neutral, focusing first on the action.

Greek word order is relatively flexible; what matters is that μέχρι τις δέκα clearly modifies the time by which the action is completed.

Why is it τη βαλίτσα μου and not την βαλίτσα μου?

In modern standard spelling, the feminine article in the accusative singular is:

τη (short form)
την (with final -ν, used only before certain sounds)

The usual rule: keep the -ν only when the following word starts with:

a vowel (α, ε, η, ι, ο, υ, ω)
or certain consonants: κ, π, τ, ξ, ψ, μπ, ντ, γκ, τσ, τζ (and combinations starting with κ, π, τ)

Βαλίτσα starts with β, which does not belong to that group, so the standard spelling is:

τη βαλίτσα μου, not την βαλίτσα μου

In speech, many people pronounce it almost the same; the orthographic rule is what makes the difference in writing.

Why does μου come after βαλίτσα? Could we say μου βαλίτσα?

The weak possessive pronoun μου (my) in Greek normally follows the noun:

η βαλίτσα μου = my suitcase
το σπίτι σου = your house
το βιβλίο του = his / its book

You cannot say μου βαλίτσα in standard Greek.

In the sentence:

τη βαλίτσα μου = my suitcase (as a direct object)

So the pattern is: article + noun + possessive
Here: τη + βαλίτσα + μου.

Why don’t we see η before βαλίτσα here? Shouldn’t it be η βαλίτσα μου?

The article η is the nominative (subject) form.
Here, βαλίτσα is not the subject; it is the direct object of the verb:

(Εγώ) θα έχω ετοιμάσει τη βαλίτσα μου.

So we need the accusative article:

η βαλίτσα (subject – nominative)
τη βαλίτσα (object – accusative)

Because βαλίτσα is the object, we use τη instead of η.

What is the base verb of ετοιμάσει, and what form is it?

The base verb is ετοιμάζω = to prepare, to get ready, to pack (a suitcase).

From this verb:

ετοιμάζω → present/imperfective stem (I prepare / I am preparing)
ετοίμασα → aorist (simple past: I prepared)
ετοιμάσω → aorist (subjunctive/future stem: to prepare (once))
έχω ετοιμάσει → present perfect: I have prepared

In θα έχω ετοιμάσει, the word ετοιμάσει is the perfective form that appears with έχω to form the perfect tenses:

έχω ετοιμάσει – I have prepared
θα έχω ετοιμάσει – I will have prepared

Can I use a simpler future instead of the future perfect? Is Μέχρι τις δέκα θα ετοιμάσω τη βαλίτσα μου acceptable?

Yes, it’s grammatically acceptable, and many native speakers would say it.

Μέχρι τις δέκα θα ετοιμάσω τη βαλίτσα μου.

In context, listeners will usually understand that you mean the suitcase will be ready by ten.

However:

Θα ετοιμάσω focuses on the action taking place in the future.
Θα έχω ετοιμάσει more clearly emphasizes that, seen from 10 o’clock, the packing will already be finished.

So the original θα έχω ετοιμάσει is a bit more precise and “textbook correct” for “by ten I’ll have…”.

What is the role of μέχρι here? Is it like English “by” or “until”?

Μέχρι can correspond to both English “until” and “by”, depending on the context.

In this sentence:

Μέχρι τις δέκα θα έχω ετοιμάσει τη βαλίτσα μου.

it means “by ten o’clock” (deadline in the future).

General uses:

With a continuing action:
Δούλευα μέχρι τις πέντε. – I was working until five.
With a deadline (especially with future perfect or similar):
Μέχρι τις πέντε θα έχω τελειώσει. – By five I will have finished.

So here μέχρι introduces the future time limit by which the action is completed.

Are there synonyms for ετοιμάσει τη βαλίτσα μου that a native might also use?

Yes, some common alternatives are:

θα έχω φτιάξει τη βαλίτσα μου
(φτιάχνω = to make / fix / put together; here: pack my suitcase)
θα την έχω ετοιμάσει (using a pronoun instead of repeating τη βαλίτσα)
e.g. Μέχρι τις δέκα θα την έχω ετοιμάσει.

All of these would sound natural in everyday Greek, with slightly different levels of formality or style but the same basic meaning.