Learn Hardcore Russian: Если у ребёнка снова будет высокая температура, нужно вызвать врача. - If the child has a high fever again, you need to call a doctor.

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

Why is it у ребёнка, not ребёнок?

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:

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

Why is будет used here? In English we usually say If the child has..., not If the child will have...

This is a major difference between English and Russian.

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

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

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.

Why is высокая температура in the nominative case?