Voda mi je do koljena.

Mi is the unstressed dative form of ja (I / me).

Here it does not mean to me in a very literal English sense. In Croatian, the dative pronoun is often used to show whose body part or personal situation is involved. So mi here means something like:

• to me
• or more naturally in English, my

So Voda mi je do koljena is literally something like The water is up to the knee(s) to me, but naturally it means The water is up to my knees.

Je is the 3rd person singular present tense of biti (to be).

Croatian often uses to be in situations where English might use a different expression, such as:

• The water is up to my knees
• The water comes up to my knees

So in this sentence, je links voda with the phrase do koljena.

A very literal structure is:

• Voda = water
• mi je = is to me / is for me
• do koljena = up to the knee(s)

Do means up to, as far as, or reaching to.

In this sentence, it marks the level that the water reaches:

• do koljena = up to the knees / knee-high

This is a very common Croatian pattern for heights and limits, for example:

• do vrata = up to the neck
• do gležnja = up to the ankle
• do pasa = up to the waist

So do is the key word that gives the idea of reaching a certain level.

Because do requires the genitive case.

The dictionary form is:

• koljeno = knee

After do, the noun changes form:

• do koljena

That is the normal thing Croatian does after many prepositions: the preposition determines the case of the noun.

So a learner should remember:

• do + genitive

This is a very common learner question.

The form koljena can look confusing because it matches more than one grammatical form. Croatian neuter nouns often have forms that look the same in different cases and numbers.

For learning purposes, the important thing is this:

• do koljena is the normal expression for up to the knees / knee-high

So even if the form looks like a plural to you, you should learn the whole phrase as a natural unit.

In other words, don't get too stuck on the appearance of the ending here. Native speakers simply use do koljena to express that height level.

Because Croatian usually does not repeat possession the way English does with body parts.

English says:

• up to my knees

Croatian very often prefers:

• mi
  • body-part expression

So instead of saying something like do mojih koljena, Croatian normally uses the shorter and more natural pattern:

• Voda mi je do koljena

This is extremely common with body parts and personal states.

Compare the idea:

• Boli me glava. = My head hurts.
  Literally: The head hurts me.

So mi is doing the job that my often does in English.

Because mi and je are clitics: short unstressed words that usually go near the beginning of the sentence, often in the second position.

So after the first stressed element, Croatian tends to place clitics such as:

• mi
• ti
• ga
• je
• se

That is why Voda mi je do koljena sounds normal.

A learner may expect something more like a word-for-word English order, but Croatian word order is more flexible, and clitic placement is an important rule.

Other word orders are possible for emphasis, but Voda mi je do koljena is the neutral, natural version.

Yes, sometimes.

• Voda je do koljena.

This means something like The water is knee-high or The water is up to the knees in a general sense.

But mi is used when you want to make it personal:

• Voda mi je do koljena. = The water is up to my knees.

So the version with mi is more specific and tied to the speaker.

Yes, but it changes the tone.

• mi = unstressed, neutral, normal
• meni = stressed, emphatic

So:

• Voda mi je do koljena. = neutral
• Voda je meni do koljena. = emphasizes me, as if contrasting with someone else

For example, you might use meni if you mean:

• The water comes up to my knees, but only to your ankles.

In ordinary speech, mi is the usual choice here.

It is both literal and idiomatic in the sense that it is a normal Croatian way to describe water depth.

Croatian commonly expresses this idea with:

• X je do + body part

So Voda mi je do koljena is a very natural way to say:

• The water is up to my knees
• The water comes up to my knees
• The water is knee-deep (depending on context)

So yes, this is exactly the kind of sentence native speakers would naturally say.