Αν πιω καπουτσίνο μετά τις έξι, η καφεΐνη με κρατάει ξύπνιο και δεν κοιμάμαι καλά.

After αν in this kind of sentence, Greek normally uses the subjunctive.
πιω is the aorist subjunctive of πίνω.

Here, αν πιω means if I drink on a particular occasion. It does not mean past time.

• αν πιω καπουτσίνο = if I drink a cappuccino
• αν πίνω καπουτσίνο = if I am drinking cappuccino / if I drink cappuccino habitually

So πιω fits better because the sentence is talking about a single possible event that leads to a result.

Because in Modern Greek, the aorist stem does not automatically mean past. What matters is the whole construction.

• ήπια = I drank → past
• να πιω = to drink / that I drink
• αν πιω = if I drink

So πιω here is not a past tense. It is an aorist subjunctive, and the aorist idea is about the action as a whole event, not about past time.

Yes, you could say έναν καπουτσίνο, but the version without it is also natural.

• να πιω καπουτσίνο = to drink cappuccino / to have cappuccino
• να πιω έναν καπουτσίνο = to drink a cappuccino / one cappuccino

Without έναν, the sentence sounds a bit more general, focusing on the type of drink rather than counting it as one item. Both are possible.

Greek usually uses the article with clock times.

• στις έξι = at six
• μετά τις έξι = after six

The article is there because Greek treats clock times as a regular noun phrase. You can think of it as something like after the six o’clock hour, though that is not how it would be translated in English.

You may also hear μετά από τις έξι. That is also correct.

Greek uses the definite article much more often than English, including with generic or abstract nouns.

So:

• η καφεΐνη με κρατάει ξύπνιο = caffeine keeps me awake

Even though English usually says just caffeine, Greek naturally says the caffeine in this kind of general statement.

Με means me. It is the object pronoun.

• η καφεΐνη = the caffeine → subject
• με = me → object
• κρατάει = keeps

So literally:

• η καφεΐνη με κρατάει ξύπνιο = the caffeine keeps me awake

In Greek, these short object pronouns usually come before the verb:

• με βλέπει = he/she sees me
• με ξέρει = he/she knows me
• με κρατάει = it keeps me

Yes. Ξύπνιο agrees with the person being kept awake, which here is με = me.

If the speaker is male:

• με κρατάει ξύπνιο

If the speaker is female:

• με κρατάει ξύπνια

So ξύπνιος / ξύπνια / ξύπνιο is an adjective, and it changes form to agree in gender and number.

That is very common in Greek. Some verbs have middle/passive endings but an active meaning.

Κοιμάμαι simply means I sleep. It is not passive here.

Compare:

• κοιμάμαι = I sleep
• κοιμίζω κάποιον = I put someone to sleep

So you just need to learn κοιμάμαι as the normal verb for sleep.

Because the sentence describes a general, repeated result, not just one specific future situation.

• Αν πιω καπουτσίνο μετά τις έξι, η καφεΐνη με κρατάει ξύπνιο και δεν κοιμάμαι καλά.

This means something like: Whenever/if I drink cappuccino after six, caffeine keeps me awake and I don’t sleep well.

If you wanted to talk about one specific future case, Greek could use future forms in the result:

• Αν πιω καπουτσίνο μετά τις έξι, δεν θα κοιμηθώ καλά.
• If I drink cappuccino after six, I won’t sleep well.

So the present tense here gives a more habitual or general truth feeling.

Yes. Both are correct.

• κρατάει
• κρατά

These are two common forms of the same verb in the present tense.
Κρατάει is often a little fuller-sounding, while κρατά is shorter. In everyday Greek, both are very normal.

So you could also say:

• η καφεΐνη με κρατά ξύπνιο

The two dots show that the ι is pronounced in a separate syllable and does not combine with the previous vowel.

So καφεΐνη is pronounced roughly:

• ka-fe-EE-ni

Without the dots, a learner might try to read the vowels together. The diaeresis helps show the correct pronunciation.