Όταν νυστάζω, πλένω το πρόσωπό μου με κρύο νερό.

Yes. νυστάζω is a very common Greek verb meaning to feel sleepy / to be getting sleepy.

A native English speaker may expect something like I am sleepy, with an adjective, but Greek often uses this verb instead.

So:

• νυστάζω = I feel sleepy
• όταν νυστάζω = when I feel sleepy / whenever I get sleepy

You can also hear adjective-based expressions in Greek, but νυστάζω is extremely natural in everyday speech.

Here όταν means when, but in this kind of sentence it often has the sense of whenever too.

Because both verbs are in the present tense, the sentence describes a habitual/repeated action:

• Όταν νυστάζω, πλένω...
• When/Whenever I get sleepy, I wash...

So this is not just about one single moment. It describes what the speaker generally does in that situation.

Because the sentence expresses a general habit.

• νυστάζω = I get sleepy / I feel sleepy
• πλένω = I wash

In Greek, the present tense is commonly used for repeated actions and routines, just like in English:

• When I feel sleepy, I wash my face...

If the speaker were talking about one specific past event or one future event, the tense choice would be different.

Greek usually omits subject pronouns when they are not needed.

The verb endings already show the subject:

• νυστάζω = I feel sleepy
• πλένω = I wash

So εγώ is unnecessary unless you want emphasis or contrast, for example:

• Εγώ πλένω το πρόσωπό μου, αλλά αυτός πίνει καφέ.
• I wash my face, but he drinks coffee.

In your sentence, the plain form without εγώ is the most natural.

Because the sentence begins with a subordinate clause:

• Όταν νυστάζω = subordinate clause
• πλένω το πρόσωπό μου με κρύο νερό = main clause

In Greek, when a clause like this comes first, it is very common to separate it with a comma.

So the comma helps mark the structure:

• When I feel sleepy, I wash my face with cold water.

Greek often uses the active verb with a body part as the direct object:

• πλένω το πρόσωπό μου = I wash my face

This is a very natural Greek pattern.

A learner may expect a reflexive structure like I wash myself, but Greek does not always match English in that way. With body parts, Greek commonly says:

• πλένω τα χέρια μου = I wash my hands
• βουρτσίζω τα δόντια μου = I brush my teeth
• πλένω το πρόσωπό μου = I wash my face

You may hear other reflexive-style expressions in Greek in some contexts, but here πλένω το πρόσωπό μου is the normal choice.

In Greek, possessive expressions usually take the definite article:

• το πρόσωπό μου = my face
• τα χέρια μου = my hands
• το βιβλίο μου = my book

This is different from English, where my usually replaces the. In Greek, the normal structure is:

• article + noun + possessive pronoun

So:

• το πρόσωπό μου literally looks like
• the face my

but it simply means

• my face

μου is the weak form of the pronoun meaning my / to me / me, depending on context.

In this sentence, it is a possessive genitive:

• το πρόσωπό μου = my face

So here μου does not mean to me. It shows possession.

This same little word appears in many uses:

• μου δίνει = he gives me
• το σπίτι μου = my house

So the function depends on the sentence.

This is due to Greek accent rules when a noun is followed by an unstressed clitic like μου.

The basic noun is:

• το πρόσωπο

But when you add μου, Greek often writes an extra accent shift to preserve the natural stress pattern:

• το πρόσωπό μου

This is a normal spelling feature with possessive clitics such as:

• ο άνθρωπός μου
• το τηλέφωνό μου
• το πρόσωπό μου

So this is not a different word—just the same noun written according to Greek accentuation rules.

Here με means with and shows the means/instrument used for the action.

So:

• με κρύο νερό = with cold water

This is very straightforward and works much like English with in this sentence.

Because νερό is a neuter singular noun, so the adjective must agree with it.

Greek adjectives change form to match the noun’s:

• gender
• number
• case

Here:

• νερό = neuter singular
• therefore κρύο = neuter singular form of cold

So:

• κρύο νερό = cold water

Compare:

• κρύος καφές = cold coffee / iced coffee masculine
• κρύα σούπα = cold soup feminine
• κρύο νερό = cold water neuter

Without the article, με κρύο νερό means with cold water in a general sense.

That is the natural choice here because the speaker is talking about the type of water used, not a specific previously identified water.

If you said:

• με το κρύο νερό

that would sound more like with the cold water, referring to some specific cold water already known from the context.

So in this sentence, no article is the normal option.

Yes. Greek word order is more flexible than English word order.

The given sentence is very natural:

• Όταν νυστάζω, πλένω το πρόσωπό μου με κρύο νερό.

But you could also say:

• Πλένω το πρόσωπό μου με κρύο νερό όταν νυστάζω.

Both are grammatical. The version with Όταν νυστάζω at the beginning highlights the condition first: When I feel sleepy...

So the original wording is especially good if the speaker wants to set up the situation before giving the action.

In this sentence, it describes a repeated/habitual action.

Greek present tense can mean:

• something happening now
• something that happens regularly
• a general habit

Because of όταν νυστάζω, the meaning is habitual:

• Whenever I feel sleepy, I wash my face with cold water.

So the present tense here is not mainly I am washing right now, but this is what I do.

Yes, πρόσωπο is the standard word for face in neutral, everyday Greek.

So:

• το πρόσωπό μου = my face

Depending on context, Greek may use other words in idiomatic expressions, but for the physical face in a sentence like this, πρόσωπο is exactly the expected word.