Το πλάσμα κοιμάται.

Because πλάσμα is a neuter noun.

In Modern Greek singular definite articles are:

• ο for masculine
• η for feminine
• το for neuter

So:

• ο άνθρωπος = the man / the person
• η γυναίκα = the woman
• το πλάσμα = the creature / the being

Grammatical gender in Greek is a property of the noun, and the article has to match it.

It is nominative singular.

That makes sense because πλάσμα is the subject of the sentence: it is the thing doing the sleeping.

So:

• το πλάσμα = nominative singular, the creature as subject

Πλάσμα literally means something like creature, being, or created thing.

Depending on context, it can sound:

• neutral: creature / being
• literary: created being
• affectionate: sometimes something like poor thing or little thing

In this sentence, the most straightforward reading is simply creature or being.

Because the verb for sleep is κοιμάμαι in its dictionary form.

This is important: κοιμάμαι looks like a middle/passive-form verb, but in Modern Greek it commonly has the active meaning sleep.

So the forms are:

• κοιμάμαι = I sleep / I am sleeping
• κοιμάσαι = you sleep
• κοιμάται = he/she/it sleeps

So κοιμάται is the correct 3rd person singular form to go with το πλάσμα.

It can mean either one, depending on context.

The Greek present tense often covers both:

• simple present: sleeps
• progressive present: is sleeping

So Το πλάσμα κοιμάται can mean:

• The creature sleeps
• The creature is sleeping

Context tells you which is more natural.

A helpful approximation is:

to PLAZ-ma kee-MA-te

A few notes:

• πλάσμα is stressed on the first syllable: PLAZ-ma
• κοιμάται is stressed on μά: kee-MA-te
• In Modern Greek, οι sounds like ee
• The ending -αι sounds like e
• The σμ in πλάσμα is often pronounced with a z-like sound before m, so it may sound close to plazma

Because κοιμάται already means sleeps / is sleeping by itself.

In English, is sleeping has two words:

• is
• sleeping

In Greek, one verb form can express the whole idea:

• κοιμάται = sleeps / is sleeping

So nothing is missing; the verb already contains the person and number information.

Not completely. Greek word order is more flexible than English.

Το πλάσμα κοιμάται is a very natural, neutral order: subject + verb.

But Greek can also move things around for emphasis, for example:

• Κοιμάται το πλάσμα.

That could sound more marked, poetic, or context-driven. For a basic neutral sentence, Το πλάσμα κοιμάται is the safest pattern.

Usually not if you mean the creature.

• Το πλάσμα κοιμάται = The creature is sleeping
• Ένα πλάσμα κοιμάται = A creature is sleeping

Greek and English do not always use articles in exactly the same way, but in this sentence the article is normal and expected.

Both the article/noun and the verb have to change.

Singular:

• Το πλάσμα κοιμάται.

Plural:

• Τα πλάσματα κοιμούνται.

So:

• το → τα
• πλάσμα → πλάσματα
• κοιμάται → κοιμούνται

This is a good example of agreement: the verb changes to match a plural subject.