Если у ребёнка снова будет высокая температура, нужно вызвать врача.

Russian often expresses to have with the pattern у + Genitive.

So instead of saying something like ребёнок имеет высокую температуру, Russian normally says:

• У ребёнка высокая температура
  literally: At the child there is a high temperature

Here:

• ребёнка is the genitive singular of ребёнок
• у ребёнка means the child has

This is a very common Russian structure for possession and states, especially with things like illnesses, symptoms, age, and availability.

This is a major difference between English and Russian.

In English, after if, we usually use the present for future meaning:

• If the child has a high temperature again, ...

In Russian, for a real future condition, you normally use the future tense:

• Если у ребёнка снова будет высокая температура...

So будет is completely normal here. Russian does not follow the English rule of avoiding future after if.

Compare:

• Если будет дождь, мы останемся дома.
  If it rains / If it is rainy, we will stay home.

Because in this type of sentence, температура is the grammatical subject.

The structure is literally:

• У ребёнка будет высокая температура
  At the child there will be a high temperature

So:

• высокая температура = nominative subject
• у ребёнка = the person affected, in у + genitive

That is why высокая agrees with температура in the nominative feminine singular.

Because вызвать takes the accusative case, and врач is an animate masculine noun.

For animate masculine nouns in Russian:

• nominative: врач
• accusative: врача

So:

• вызвать врача = to call a doctor

This is a very important pattern:

• Я вижу брата
• Мы ждём учителя
• Нужно вызвать врача

All of these use the accusative, but the form looks like the genitive because the noun is animate.

Нужно creates an impersonal construction meaning:

• it is necessary
• one should
• it’s necessary to

So нужно вызвать врача means:

• it’s necessary to call a doctor
• a doctor should be called

Russian often leaves the person unspecified when it is obvious or unimportant.

If you wanted to name who needs to do it, you could say:

• Нам нужно вызвать врача. = We need to call a doctor.
• Вам нужно вызвать врача. = You need to call a doctor.

But without that, it stays general and impersonal.

Because вызвать is perfective, and here the sentence refers to one complete action: calling the doctor if the condition happens.

• вызвать = to call/summon successfully, as a completed action
• вызывать = to be calling / to call repeatedly / to call in a more process-oriented or habitual sense

After нужно, Russian often uses the perfective infinitive when the idea is:

• a single necessary action
• do this once, as needed

So:

• нужно вызвать врача = it is necessary to call a doctor

If you used вызывать, it would sound more like repeated or general activity, and it would be less natural here.

Yes. Снова means again, and in this sentence опять would also be possible.

• снова = again
• опять = again

In many contexts they are interchangeable.

So these are both natural:

• Если у ребёнка снова будет высокая температура...
• Если у ребёнка опять будет высокая температура...

Very roughly:

• снова can sound a bit more neutral or formal
• опять can sometimes sound a bit more conversational, and sometimes can carry a tone like again, unfortunately

But in this sentence, both work.

Because the sentence begins with a subordinate clause introduced by если.

Structure:

• Если у ребёнка снова будет высокая температура,
  нужно вызвать врача.

In Russian, subordinate clauses are separated by commas, and this punctuation is much more regular and mandatory than in English.

So whenever you have a clause with если attached to a main clause, you normally need a comma.

Yes, Russian word order is flexible, though the original order is very natural and neutral.

Neutral version:

• Если у ребёнка снова будет высокая температура, нужно вызвать врача.

Possible variations:

• Если снова у ребёнка будет высокая температура, нужно вызвать врача.
• Нужно вызвать врача, если у ребёнка снова будет высокая температура.

These versions are still understandable, but they shift emphasis a little.

The original sentence is good because:

• the condition comes first
• снова is placed naturally
• the recommendation follows clearly

So it is probably the best version for standard usage.

Yes, ребёнка is a genitive form, so by itself it could appear in different contexts. But the pattern у + genitive is a very strong signal.

When you see:

• у ребёнка

you should often think:

• the child has
• with the child
• in the child’s case

The exact English translation depends on context.

Here, because it is followed by будет высокая температура, the meaning is clearly:

• the child will have a high temperature

So the whole chunk у ребёнка будет works together as a possession/state expression.

Not in this sentence, because the meaning is future.

Russian can omit the present-tense form of to be:

• У ребёнка высокая температура. = The child has a high temperature.

But in the future, you must use будет:

• У ребёнка будет высокая температура.

So:

• present: no есть
• future: будет is required

That is why Если у ребёнка снова будет высокая температура... is correct.

Yes, it is very normal.

Russian commonly says:

• высокая температура = high temperature / fever
• у ребёнка температура = the child has a temperature
• поднялась температура = the temperature went up / the child developed a fever

So the phrase in the sentence is natural and idiomatic.

In everyday speech, Russians may also say:

• Если у ребёнка снова будет температура, ...

even without высокая, because the context already suggests illness. But высокая температура is more explicit.

The word is spelled ребёнка, with ё, and the stress falls there:

• ре-БЁН-ка

The letter ё always indicates stress.

This matters because:

• ребёнок = child
• ребенка is often written without the dots in ordinary text, but it is still understood as ребёнка

In printed Russian, ё is often replaced by е, especially in non-learner texts, so you may see:

• Если у ребенка снова будет высокая температура...

But the pronunciation and the correct dictionary form are still with ё:

• ребёнок
• ребёнка