Коли в дитини нежить, ніс треба витирати м’якою серветкою.

Ukrainian very often expresses possession with the pattern у/в + genitive + noun.

So в дитини нежить is literally something like at the child, there is a runny nose, and naturally means the child has a runny nose.

You can also say дитина має нежить, but у/в дитини нежить is a very common, natural way to say it.

Because after у/в in this possession pattern, the noun goes into the genitive case.

• дитина = nominative
• дитини = genitive singular

So:

• дитина = a child
• в дитини = in/at the child -> idiomatically the child has

In the present tense, Ukrainian often leaves out є (is / there is).

So the full underlying idea is:

• У дитини є нежить

But in normal speech and writing, є is often omitted:

• У дитини нежить

That is completely normal.

Нежить means a runny nose or nasal congestion/catarrh. It refers to the symptom, not necessarily the whole illness.

So it is narrower than English a cold. A child can have нежить as one symptom.

Here ніс is the direct object of витирати (to wipe), so it is in the accusative case.

For many masculine inanimate nouns in Ukrainian, the accusative singular looks exactly like the nominative singular.

So:

• nominative: ніс
• accusative: ніс

That is why you see витирати ніс = to wipe the nose.

Треба means it is necessary, one should, or need to.

In this sentence, it gives general advice:

• ніс треба витирати = the nose should be wiped / you should wipe the nose

It is an impersonal word, so Ukrainian does not need a subject like you or one.

This is about aspect.

• витирати = imperfective
• витерти = perfective

The imperfective витирати is used because the sentence gives general advice or describes a repeated action: whenever the child has a runny nose, you should wipe the nose.

If you used витерти, it would sound more like wipe it once / complete the action.

So:

• треба витирати = should be wiping / should wipe regularly
• треба витерти = need to wipe (once, to completion)

Because this phrase means with a soft tissue/napkin, and Ukrainian uses the instrumental case for the thing used to perform an action.

So both words are in the instrumental singular:

• м’яка серветка = a soft tissue/napkin
• м’якою серветкою = with a soft tissue/napkin

This is a very common pattern:

• писати ручкою = to write with a pen
• різати ножем = to cut with a knife
• витирати серветкою = to wipe with a tissue/napkin

It can, depending on context.

The basic meaning of серветка is often napkin or serviette, but in everyday contexts it can also refer to something used for wiping, including a tissue-like wipe.

In this sentence, because it says м’якою серветкою and the context is wiping a child’s nose, English will often translate it naturally as a soft tissue.

If you want to be more explicit about a disposable paper tissue, Ukrainian might also say:

• паперовою хустинкою = with a paper tissue
• носовою хустинкою = with a handkerchief

Literally, коли means when.

But in general advice sentences like this, English sometimes uses if or when:

• When a child has a runny nose...
• If a child has a runny nose...

Ukrainian коли here has the sense of whenever that situation happens. So when is the direct translation, but if may sound natural in English depending on style.

Yes. У and в often alternate in Ukrainian for euphony—to make the sentence sound smoother.

So both are possible:

• Коли в дитини нежить...
• Коли у дитини нежить...

The meaning does not change. The choice often depends on surrounding sounds and speaking rhythm.

Yes. Ukrainian word order is fairly flexible.

For example, you could also say:

• Коли у дитини нежить, треба витирати ніс м’якою серветкою.

That has basically the same meaning.

The original sentence puts ніс earlier, which gives it a little more focus. English is usually much less flexible with word order than Ukrainian.

Because Коли в дитини нежить is a subordinate clause introduced by коли.

So the structure is:

• Коли в дитини нежить, = subordinate clause
• ніс треба витирати м’якою серветкою. = main clause

Ukrainian, like English, separates this kind of introductory subordinate clause with a comma.