Ono što mi doktorica uvijek ponavlja jest da prehlada prolazi brže ako dovoljno spavam i pijem mnogo vode.

Ono što is a very common Croatian way to say what in the sense of the thing that.

So:

• Ono = that / it / the thing
• što = which / what / that

Together, Ono što mi doktorica uvijek ponavlja... means something like:

• What my doctor always repeats to me...
• literally: That which my doctor always repeats to me...

This structure is very normal in Croatian when introducing a clause that functions as the subject of the sentence.

Mi here means to me.

It is the unstressed dative form of ja. So:

• ja = I
• meni = to me
• mi = to me (short/clitic form)

In this sentence:

• doktorica mi ponavlja = the doctor repeats to me

Croatian often uses these short pronoun forms very early in the sentence, usually in second position. That is why mi comes right after što.

Doktorica means female doctor.

Croatian often marks gender in profession nouns:

• doktor = male doctor
• doktorica = female doctor

So the sentence specifically tells you the doctor is a woman. If the doctor were male, it would be:

• Ono što mi doktor uvijek ponavlja...

Uvijek means always.

In Croatian, adverbs like uvijek are often placed before the verb:

• doktorica uvijek ponavlja = the doctor always repeats

That word order is very natural. Croatian word order is more flexible than English, but this placement is standard and easy to understand.

Ponavlja means repeats.

It comes from the verb ponavljati (ponavljati se / ponoviti is the perfective partner, depending on use). Here the important point is that ponavljati is imperfective, which fits the idea of repeated or habitual action:

• doktorica uvijek ponavlja = the doctor always keeps repeating / always repeats

Because uvijek describes a habitual action, the imperfective verb is exactly what you would expect.

Jest is the full, stressed form of je (is).

So:

• je = unstressed is
• jest = stressed/full is

In sentences like this, Croatian often uses jest to clearly connect two parts of the sentence:

• Ono što mi doktorica uvijek ponavlja jest da...
• What my doctor always repeats is that...

Using jest sounds a bit more formal or careful than je. In everyday speech, many speakers might also say je, but jest is very common in this kind of structure.

Here da introduces a content clause, similar to English that:

• jest da prehlada prolazi brže...
• is that a cold passes/goes away faster...

Croatian uses da very often where English uses that to introduce what someone says, knows, thinks, repeats, etc.

So the structure is:

• Ono što ... jest da ...
• What ... is that ...

Prehlada means a cold (the illness).

In the clause:

• prehlada prolazi brže

the subject is prehlada, because it is the thing doing the action of prolazi:

• the cold passes / goes away faster

So literally, Croatian says something like:

• a cold passes faster

where English often prefers:

• a cold goes away faster
• you get over a cold faster

Prolazi literally means passes or goes by, but with illnesses it means goes away / passes.

So:

• prehlada prolazi = the cold goes away / passes
• prehlada prolazi brže = the cold goes away faster

Brže is the comparative form of brzo:

• brzo = fast / quickly
• brže = faster / more quickly

Even though English might say I recover faster, Croatian naturally says the cold passes faster.

Ako means if, and in Croatian real or likely conditions often use the present tense in both parts of the sentence.

So:

• ako dovoljno spavam i pijem mnogo vode
• literally: if I sleep enough and drink a lot of water

This is normal Croatian grammar. English also often uses the present after if:

• if I sleep enough
• not if I will sleep enough

So this part matches English fairly well.

Because the sentence says if I sleep enough and drink a lot of water. These are finite verbs in the 1st person singular present tense:

• spavam = I sleep
• pijem = I drink

Croatian uses full conjugated verbs here because the subject is understood to be I.

If you wanted infinitives, you would need a different structure, such as after another verb. But after ako, Croatian normally uses a conjugated verb.

Dovoljno means enough.

In:

• dovoljno spavam

it means:

• I sleep enough
• more naturally in English: I get enough sleep

Croatian often uses spavati dovoljno where English might prefer get enough sleep. Both express the same idea.

Because mnogo usually requires the following noun to be in the genitive.

So:

• voda = water (nominative)
• vode = water (genitive singular)

Therefore:

• mnogo vode = a lot of water

This is a very common pattern in Croatian:

• puno vremena = a lot of time
• mnogo ljudi = many people / a lot of people
• mnogo vode = a lot of water

So vode is not plural here; it is genitive singular after mnogo.

Because Croatian usually drops subject pronouns when they are clear from the verb form.

The endings already tell you who is doing the action:

• spavam = I sleep
• pijem = I drink

So there is no need to say ja unless you want emphasis or contrast.

For example:

• ako dovoljno spavam i pijem mnogo vode = normal
• ako ja dovoljno spavam i pijem mnogo vode = more emphatic, like if I get enough sleep...

Croatian is a pro-drop language, unlike English.

Yes. A very natural English version would be:

• What my doctor always tells me is that a cold goes away faster if I get enough sleep and drink plenty of water.

A more literal version is:

• What my female doctor always repeats to me is that a cold passes faster if I sleep enough and drink a lot of water.

Both are fine, but the first sounds more natural in English.

Croatian word order is fairly flexible, but this version is natural and standard.

The sentence is organized like this:

1. Ono što mi doktorica uvijek ponavlja
   = What my doctor always repeats to me

2. jest
   = is

3. da prehlada prolazi brže ako dovoljno spavam i pijem mnogo vode
   = that a cold goes away faster if I sleep enough and drink a lot of water

Some parts could move around in other contexts, especially adverbs and clitics, but this sentence is already in a very natural order for neutral written Croatian.