Ο γιατρός είπε να παίρνω βιταμίνη κάθε μέρα και να πίνω περισσότερο νερό.

Because Ο γιατρός is the subject of είπε, but the subject of να παίρνω is different and is understood from context as I.

So the structure is roughly:

• Ο γιατρός είπε = the doctor said
• να παίρνω... = that I should take / for me to take...

Greek often leaves out subject pronouns like εγώ when the verb ending already shows the person. The ending -ω in παίρνω tells you it is I.

In this kind of sentence, είπε να means something like:

• said that I should...
• said to...
• sometimes, depending on context, told me to...

The word να introduces the action that was said, suggested, or instructed. It is a very common Greek pattern after verbs like λέω, θέλω, πρέπει, and many others.

So είπε να παίρνω... is the natural Greek way to express an idea that English often gives with to take or should take.

Because Modern Greek does not normally use an infinitive the way English does. Instead, it usually uses να + a finite verb form.

So where English says:

• to take
• to drink

Greek uses:

• να παίρνω
• να πίνω

That is one of the biggest structural differences between English and Modern Greek.

This is about aspect, which is very important in Greek.

• να παίρνω, να πίνω = imperfective aspect
  This suggests a repeated, habitual, or ongoing action.
• να πάρω, να πιω = perfective aspect
  This usually points to a single, complete action.

Here the sentence says κάθε μέρα, so the meaning is clearly habitual: the doctor said I should keep taking vitamin every day and keep drinking more water. That is why Greek uses the imperfective forms παίρνω and πίνω.

Because Greek normally repeats να before each coordinated verb in this structure:

• να παίρνω... και να πίνω...

This makes it clear that both verbs belong to the same kind of να-clause. English does something similar less visibly with repeated structures, but in Greek the repetition is very natural and standard.

It could be there, but it does not have to be.

• Ο γιατρός είπε να παίρνω... = The doctor said that I should take...
• Ο γιατρός μου είπε να παίρνω... = The doctor told me to take...

Adding μου makes the indirect object explicit: to me. Without it, the sentence can still be perfectly natural if the context already makes it clear who the advice was for.

So the version without μου is not wrong; it is just a little less explicit.

Greek often leaves a noun bare when speaking in a general or non-specific way, especially in advice or routine actions.

So παίρνω βιταμίνη can mean something like:

• take vitamin
• take a vitamin supplement
• take vitamins, in a general sense

If you said μια βιταμίνη, that would sound more like one vitamin or a particular vitamin pill/tablet.

So the bare noun βιταμίνη here sounds natural for general medical advice.

κάθε μέρα means every day.

• κάθε = every / each
• μέρα = day

Greek does not need an article in this expression. κάθε already determines the noun, so κάθε μέρα is the normal form.

You can also see κάθε ημέρα, which is a bit more formal, but κάθε μέρα is the everyday version.

Yes. Here περισσότερο is the neuter singular form, and it matches νερό, which is also neuter singular.

• περισσότερος = masculine
• περισσότερη = feminine
• περισσότερο = neuter

Since νερό is neuter, Greek uses περισσότερο νερό = more water.

This is a good example of adjective-like agreement in Greek. You might also hear πιο πολύ νερό, which means the same thing and is very common in speech.

It can overlap with both, depending on context.

Strictly speaking:

• είπε = said
• είπε να... often works like said that someone should...
• with μου or another indirect object, it often feels very close to told me to...

So in this sentence, English may translate it naturally as either:

• The doctor said I should...
• The doctor told me to...

The exact best choice depends on the context and the translation style.