É importante que você fique tranquilo durante a prova.

In Portuguese, expressions like É importante, É necessário, É bom, É melhor, É fundamental often introduce a clause with que meaning that:

• É importante que… = It’s important that…
  So que is the connector that links the impersonal evaluation (it’s important) to the action you want someone to do.

Because É importante que… typically triggers the present subjunctive in Brazilian Portuguese when you’re expressing a recommendation, necessity, or importance.

• (que) você fique = present subjunctive of ficar for você
  Indicative (você fica) would sound like you’re stating a fact/habit: you stay/you end up (as a description), not advising or emphasizing what should happen.

The infinitive is ficar (to stay / to become).
Present subjunctive of ficar:

• que eu fique
• que você fique
• que ele/ela fique
• que nós fiquemos
• que vocês fiquem
• que eles/elas fiquem

So fique matches você (and also eu/ele/ela in form, but the subject here is você).

Both can translate as to be calm, but the nuance differs:

• ficar tranquilo often means to become calm or to stay/keep calm (a change or an instruction to maintain calm).
• estar tranquilo is more simply to be calm (describing a state).

In advice contexts, ficar tranquilo is very common: (It’s important that) you stay calm.

In most of Brazil, você is the default informal/neutral you, and it takes 3rd-person verb forms (like você fique).
In regions where tu is common, you might hear:

• É importante que tu fiques tranquilo durante a prova.
  But many Brazilians who say tu still use 3rd-person verbs informally (tu fica), depending on the region.

Yes—adjectives agree in gender and number:

• to a man: você fique tranquilo
• to a woman: você fique tranquila
• to a group (mixed/men): vocês fiquem tranquilos
• to a group (all women): vocês fiquem tranquilas

Often, yes, but the meaning shifts slightly:

• durante a prova = during the exam (focus on the time period while it’s happening)
• na prova = in the exam / on the test (can mean during it, but can also lean toward the setting or the context of the exam)

Both are common; durante is more explicitly about time.

It’s an impersonal structure:

• É importante… = It is important…

Portuguese often uses ser + adjective impersonally without an explicit subject, similar to English it is…. The it is understood.

Common stress points:

• importante: im-por-TAN-chi (final e often sounds like i in Brazil)
• você: vo-SÊ (the ê is stressed)
• fique: FI-ki (the que sounds like ki)
• tranquilo: tran-QUI-lo
• durante: du-RAN-chi
• prova: PRO-va

Some flexibility is possible, but the given order is the most natural:

• É importante que você fique tranquilo durante a prova.

You could move the time phrase for emphasis:

• É importante que você fique tranquilo, durante a prova. But too much rearranging can sound marked or less smooth. The core pattern É importante que + subjunctive is usually kept intact.

Yes—common variations include:

• É importante você ficar tranquilo durante a prova. (very common in speech; many Brazilians use infinitive here)
• É essencial que você mantenha a calma durante a prova. (more formal; mantenha is subjunctive of manter)
• É bom que você fique tranquilo durante a prova. (a bit softer than importante)