Ο παιδίατρος είπε ότι το μωρό πρέπει να κοιμάται περισσότερο.

In Greek, nouns usually appear with a definite article much more often than in English.

• ο παιδίατρος = the pediatrician
• το μωρό = the baby

Here the articles also show grammatical gender and case:

• ο = masculine singular nominative
• το = neuter singular nominative

So the articles are not optional decoration; they help identify how the nouns function in the sentence.

Yes, in this sentence ο makes the pediatrician masculine, so it suggests a male doctor.

However, the noun παιδίατρος itself can be used for either sex, with the article changing:

• ο παιδίατρος = the male pediatrician
• η παιδίατρος = the female pediatrician

So the noun form often stays the same, but the article tells you the gender.

No. This is grammatical gender, not biological sex.

In Greek, μωρό is a neuter noun, so it takes neuter forms:

• το μωρό
• καλό μωρό
• μικρό μωρό

This does not mean anything about the baby’s actual sex. Greek often uses neuter for words referring to babies and young children.

Είπε is the aorist form of λέω (to say / tell).
It means said.

So:

• λέει = he/she says
• είπε = he/she said

In this sentence, Ο παιδίατρος είπε... means The pediatrician said...

The aorist here gives a simple completed past action: the doctor said it.

Ότι here means that and introduces a content clause:

• είπε ότι... = said that...

So the sentence structure is:

• The pediatrician said
• that the baby should sleep more

This is very common in Greek after verbs like λέω (say), νομίζω (think), ξέρω (know), and so on.

No. They look similar but mean different things.

• ότι = that
• ό,τι = whatever / anything that

Examples:

• Μου είπε ότι θα έρθει. = He told me that he will come.
• Πάρε ό,τι θέλεις. = Take whatever you want.

In your sentence, it must be ότι, because it introduces what the pediatrician said.

Πρέπει means something like:

• must
• should
• it is necessary

It is often used impersonally in Greek. That means it does not really have a normal subject like I or he.

So:

• πρέπει να φύγω = I must leave
• πρέπει να κοιμάται περισσότερο = the baby should sleep more / it is necessary for the baby to sleep more

Literally, the Greek idea is closer to it is necessary that the baby sleep more.

Because in Modern Greek, verbs like πρέπει are followed by να + verb.

So:

• πρέπει να κοιμάται
• not πρέπει κοιμάται

A useful way to think about this is that Modern Greek usually does not use an infinitive the way English does. Instead of to sleep, Greek normally uses να plus a finite verb form.

So English:

• should sleep

becomes Greek:

• πρέπει να κοιμάται

That is a very common question. Yes, κοιμάται is the present-form stem, but after να it functions in a dependent clause, not as an ordinary standalone present tense.

Here it expresses imperfective aspect: ongoing, repeated, or habitual action.

So να κοιμάται περισσότερο means something like:

• to sleep more in general
• to be sleeping more
• to get more sleep regularly

It is not talking about one single moment. It describes the baby’s needed sleeping pattern.

This is an aspect difference:

• να κοιμάται = imperfective
• να κοιμηθεί = perfective

Very roughly:

• να κοιμάται focuses on the process, duration, or habit of sleeping
• να κοιμηθεί focuses on a single whole event, often closer to fall asleep or sleep once

So here:

• το μωρό πρέπει να κοιμάται περισσότερο = The baby should be sleeping more / should get more sleep

That is more natural than να κοιμηθεί περισσότερο, because the sentence is about the baby’s overall amount of sleep, not one single sleeping event.

Because here περισσότερο is working as an adverb, not as an adjective.

It modifies the verb κοιμάται:

• κοιμάται περισσότερο = sleeps more

Since it is adverbial, it does not agree with μωρό.

Compare:

• Το μωρό κοιμάται περισσότερο. = The baby sleeps more.
  (περισσότερο = adverb)

But:

• Το μωρό χρειάζεται περισσότερες ώρες ύπνου. = The baby needs more hours of sleep.
  (περισσότερες agrees with ώρες)

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

The sentence as written is a neutral, natural order:

• Ο παιδίατρος είπε ότι το μωρό πρέπει να κοιμάται περισσότερο.

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

• Είπε ο παιδίατρος ότι το μωρό πρέπει να κοιμάται περισσότερο.

That still means the same thing, but the emphasis or flow may change slightly. For learners, the version you have is the safest and most standard one to use.