miànshì yǐqián yào lěngjìng.

Because Chinese often makes before X by putting 以前 or 之前 after the event or time word.

So:

• 面试以前 = before the interview
• 吃饭以前 = before eating / before the meal
• 上课以前 = before class

This is a very common pattern:

• X 以前 = before X
• X 以后 = after X

It is functioning like an event noun here: the interview.

A useful thing to know is that many Chinese words can behave more flexibly than English words. 面试 can mean:

• to interview / to have an interview as a verb
• an interview as a noun-like event

In 面试以前, it refers to the event itself, so the phrase means before the interview.

Chinese often leaves out the subject when it is obvious from context.

So 面试以前要冷静 could mean:

• Before the interview, you need to stay calm
• Before an interview, one should stay calm
• Before the interview, we should stay calm

The sentence sounds like general advice, so the subject is omitted naturally.

If you want to add one, you could say:

• 你面试以前要冷静。
• 面试以前，你要冷静。

Here, 要 does not mean want to. It means something like:

• need to
• should
• must

The exact strength depends on context.

So in this sentence, 要冷静 means need to stay calm / should stay calm.

Compare:

• 我要喝咖啡。 = I want coffee / I want to drink coffee.
• 面试以前要冷静。 = Before the interview, you need to stay calm.

Same word, different meaning depending on context.

Because 冷静 here means be calm / stay calm, and after 要, Chinese can use an adjective-like word directly as the main predicate.

So:

• 要冷静 = need to be calm / need to stay calm

You do not need 很 here.

很 is often used when simply describing someone or something:

• 他很冷静。 = He is calm.

But after a modal verb like 要, the structure is different:

• 要冷静
• 应该小心
• 可以安静一点

So 要很冷静 is possible in some contexts, but it would mean need to be very calm, which is a different nuance.

Yes, it would be correct.

• 面试以前要冷静
• 在面试以前要冷静

Both are natural. The version without 在 is a little more compact.

Chinese time expressions often do not need 在 when they appear before the main verb or predicate. This is especially common in short advice sentences.

They are very close in meaning. In this sentence, both work:

• 面试以前要冷静。
• 面试之前要冷静。

Both mean before the interview, stay calm.

A small difference:

• 之前 is very specifically before
• 以前 can also mean in the past / formerly in other contexts

So 之前 is sometimes a bit clearer if you want only the time meaning before. But here, 以前 is perfectly natural.

Yes, absolutely. That is very common and often sounds even more concise.

Compare:

• 面试以前要冷静。
• 面试之前要冷静。
• 面试前要冷静。

All three are natural.

前 is shorter and very common in everyday speech:

• 考试前 = before the exam
• 睡觉前 = before sleeping
• 出门前 = before going out

So 面试前要冷静 is a very useful version to remember.

Because Chinese usually places time expressions before the main action or state.

A common word order is:

• subject + time + verb/predicate

Since the subject is omitted here, the sentence starts directly with the time phrase:

• 面试以前
  • 要冷静

This is very normal in Chinese.

You could also say:

• 面试以前，你要冷静。

That version just makes the subject clearer.

It can potentially be interpreted either way in theory, because Chinese often does not mark the noun/verb distinction as sharply as English.

But in this sentence, the most natural reading is:

• before the interview

That is because 要冷静 sounds like advice given to someone preparing for an interview.

If the context were about a company interviewing candidates, then 面试以前 could potentially be understood as before interviewing people. Context decides.

It is natural and understandable.

That said, many speakers might more often say:

• 面试前要冷静。
• 面试之前要冷静。

These sound a little more everyday and compact.

So the original sentence is fine, but if you want a very common spoken version, 面试前要冷静 is especially useful.

Yes, depending on context.

冷静 can describe a state:

• 他很冷静。 = He is calm.

And it can also be used more dynamically:

• 你要冷静！ = You need to calm down / Stay calm!

In 面试以前要冷静, it has the sense of stay calm or be calm before the interview.

So it is not just a static adjective in translation; it can behave a bit like a verb in Chinese.