Όταν έχω νύστα, δεν ξέρω τι να κάνω: να πιω άλλο καφέ ή να πάω λίγο βόλτα;

Here όταν means when / whenever.

With όταν έχω νύστα, the sense is general or habitual: whenever I feel sleepy. It is not necessarily about one single occasion only.

So the sentence sounds like: When/Whenever I’m sleepy, I don’t know what to do...

Because έχω νύστα is the normal Greek expression for I’m sleepy / I feel sleepy.

Literally, it is closer to I have sleepiness, but learners should treat it as a fixed everyday expression.

A few similar Greek patterns are:

• έχω κρύο = I’m cold
• έχω ζέστη = I’m hot
• έχω όρεξη = I feel like / I’m in the mood for

So έχω νύστα is just idiomatic Greek.

Because Greek often uses question word + να + verb where English uses an infinitive such as what to do.

So:

• δεν ξέρω τι να κάνω = I don’t know what to do

This is very common in Greek:

• δεν ξέρω πού να πάω = I don’t know where to go
• δεν ξέρω τι να πω = I don’t know what to say
• ξέρω πώς να το κάνω = I know how to do it

Since Modern Greek does not use an infinitive the way English does, να + verb often does that job.

This is an important difference.

• δεν ξέρω τι να κάνω = I don’t know what to do
• δεν ξέρω τι κάνω = I don’t know what I’m doing

So:

• τι να κάνω expresses a choice, problem, or uncertainty about a future action
• τι κάνω describes an actual action or situation

In this sentence, the speaker is trying to decide between two possible actions, so τι να κάνω is the right form.

Because Greek normally repeats να before each separate option.

So:

• να πιω άλλο καφέ ή να πάω λίγο βόλτα

This works like English:

• to drink more coffee or to go for a short walk

Greek usually sounds more natural when να is repeated for each verb in this kind of either/or choice.

Because να πιω is the perfective (aorist) subjunctive form of πίνω.

In this sentence, the speaker is considering one complete action:

• να πιω άλλο καφέ = to drink/have some more coffee, as a single act

If you used να πίνω, it would suggest something more ongoing, repeated, or habitual.

Compare:

• να πιω καφέ = to have a coffee, one act
• να πίνω καφέ = to be drinking coffee / to drink coffee regularly

Also, πίνω is irregular, so its perfective stem is πι-, which gives να πιω.

For the same reason: να πάω suggests one complete trip/action.

Here the speaker means: Should I go out for a little walk right now?

So:

• να πάω = to go once, as a complete action
• να πηγαίνω = to be going, or to go regularly/repeatedly

Compare:

• Θέλω να πάω σπίτι. = I want to go home.
• Μου αρέσει να πηγαίνω στο πάρκο. = I like going to the park.

In your sentence, a single outing is intended, so να πάω fits best.

Here άλλο καφέ means something like more coffee or another coffee, depending on context.

In everyday Greek, food and drink words often appear without an article when the meaning is indefinite or general.

So:

• να πιω άλλο καφέ = to drink some more coffee / to have another coffee

If a speaker wants to be very explicit about one more cup, they might say:

• να πιω άλλον έναν καφέ

So the version in your sentence is natural and colloquial.

βόλτα is a very common word meaning walk, stroll, or even a little outing.

The expression πάω βόλτα means:

• I go for a walk
• I go out for a bit
• I go for a stroll

It is a fixed everyday phrase, so learners should remember it as a chunk.

Examples:

• Πάμε βόλτα; = Shall we go for a walk?
• Βγήκα μια βόλτα. = I went out for a bit.

Here λίγο means a little / for a bit.

It does not mean the speaker will literally walk a small walk. Instead, it modifies the whole idea and gives the sense:

• go for a little walk
• go out for a bit

So:

• να πάω λίγο βόλτα = to go for a short walk / to go out for a bit

This is natural colloquial Greek. You may also hear similar ideas expressed as:

• να πάω μια βόλτα
• να πάω για λίγο βόλτα

Because in Greek, the semicolon ; is used as the question mark.

So:

• Greek ; = English ?

That means:

• ... ή να πάω λίγο βόλτα; is a question

Also, the : before the two options introduces the explanation or choice:

• I don’t know what to do: drink more coffee or go for a short walk?