Learn French: Il vaut mieux qu'ils ne soient pas bruyants. - It is better that they not be noisy.

We use the subjunctive mood, soient, because the sentence starts with Il vaut mieux que (It is better that). Impersonal expressions that state a preference, opinion, or necessity followed by que almost always trigger the subjunctive.

What verb is soient from, and why does it look so different?

Soient is the present subjunctive form of être (to be) for ils or elles. It is highly irregular! Instead of being based on the standard present tense, the subjunctive of être uses the special stem soi-.

What verb is used in the phrase Il vaut mieux?

It uses the verb valoir (to be worth). While the phrase translates naturally to 'It is better', its literal meaning is closer to 'It is worth better'. It is a very common fixed expression.

Why does bruyants have an 's' at the end?

Because it is an adjective describing the plural subject ils (they). Even though it comes after a subjunctive verb, the basic rule of adjective agreement still applies: adjectives must agree in gender and number with what they describe, so we add an -s to the singular form bruyant.