Όταν νυστάζω στο μάθημα, πίνω νερό και περπατάω λίγο στο διάλειμμα για να μου φύγει η νύστα.

Here it is best understood as whenever or when(ever), because the whole sentence describes a habitual situation.

So Όταν νυστάζω στο μάθημα, πίνω νερό... means something like:

Whenever I get sleepy in class, I drink water...

Greek often uses όταν + present tense for repeated situations like this.

Because the speaker is describing a general habit, not one specific event.

In English we do the same thing:

When I get sleepy in class, I drink water and walk a little...

So the Greek present here means:

• νυστάζω = I get sleepy / I feel sleepy
• πίνω = I drink
• περπατάω = I walk

They are not necessarily happening right now; they describe what usually happens.

They are related, but they are different parts of speech:

• νυστάζω = I’m sleepy / I feel drowsy
  This is a verb.
• η νύστα = sleepiness / drowsiness
  This is a noun.

So in the sentence:

• Όταν νυστάζω... = When I feel sleepy...
• να μου φύγει η νύστα = for the sleepiness to go away

This verb–noun relationship is very common in Greek.

Because στο is the contraction of σε + το.

So:

• στο μάθημα = in class / during the lesson
• στο διάλειμμα = at break / during the break

This is extremely common in Greek:

• σε + το = στο
• σε + τη = στη
• σε + τον = στον

Also, Greek often uses the article where English might not. So στο μάθημα sounds natural even though English often just says in class.

Because νερό is being used in a general, non-specific way.

So:

• πίνω νερό = I drink water

This is like English: we usually say drink water, not drink the water, unless we mean some specific water.

If Greek wanted to refer to a particular water, it could use the article:

• πίνω το νερό = I drink the water

But here the meaning is general, so νερό without an article is normal.

Here λίγο means a little or for a bit, and it modifies the verb περπατάω.

So:

• περπατάω λίγο = I walk a little / I walk for a bit

In this sentence, λίγο is functioning as an adverb, not an adjective.

Yes. Both are correct.

• περπατάω = very common in everyday speech
• περπατώ = also correct, often felt as a bit shorter or slightly more formal

So these are both fine:

• πίνω νερό και περπατάω λίγο
• πίνω νερό και περπατώ λίγο

The meaning is the same.

για να introduces a purpose clause. It means so that, in order to, or simply to.

So:

• πίνω νερό και περπατάω λίγο στο διάλειμμα για να... = I drink water and walk a little during the break so that / in order to...

After για να, Greek uses the subjunctive.

That is why the sentence continues with μου φύγει, not with an infinitive like English would.

Because να μου φύγει uses the aorist subjunctive, which expresses a single, complete result:

for the sleepiness to go away

That fits the idea perfectly: the speaker wants the drowsiness to leave or wear off.

If you said να μου φεύγει, that would be the present subjunctive, which suggests something more ongoing, repeated, or in progress. That does not fit as well here.

So:

• να μου φύγει η νύστα = so that the sleepiness goes away
• να μου φεύγει η νύστα = less natural here

μου is a weak object pronoun meaning something like from me or for me in this expression.

Literally, the phrase is close to:

for the sleepiness to leave me

That is why Greek says:

• να μου φύγει η νύστα

This is a very natural Greek structure. English would usually express the same idea less literally, for example:

• so that the sleepiness goes away
• so that I wake up a bit

So μου is important here, even though it may not translate word-for-word into natural English every time.

Yes, η νύστα is the subject of φύγει.

Literally, the structure is:

• η νύστα = the sleepiness
• φεύγει / φύγει = goes away / leaves
• μου = from me

So the literal idea is:

so that the sleepiness leaves me

Greek word order is more flexible than English, and it is very common for the subject to come after the verb. So να μου φύγει η νύστα sounds completely natural.

The word order is not completely fixed, but the original sentence sounds very natural.

Greek allows movement for emphasis, rhythm, or style. For example, you could move parts around, but some versions sound more natural than others.

The original order works well because it clearly groups the sentence into:

• Όταν νυστάζω στο μάθημα
• πίνω νερό και περπατάω λίγο στο διάλειμμα
• για να μου φύγει η νύστα

So yes, Greek has flexibility, but this version is a very normal and idiomatic way to say it.