Learn French: Que le chien soit craintif montre son traumatisme. - That the dog is fearful shows his trauma.

The phrase Que le chien soit craintif is a noun clause acting as the subject of the entire sentence (meaning "That the dog is fearful..."). In French, when you start a sentence with Que to make a clause the subject, the verb inside that clause must always be in the subjunctive. This structure frames the idea as a concept being commented on rather than just stating a standalone fact.

What is the subject of the main verb montre?

The subject is the entire clause that comes before it: Que le chien soit craintif. Because a clause acts as a single, abstract idea, the main verb (montre) is conjugated in the third-person singular, just as it would be if the subject were il or cela.

Is there a difference between using craintif and peureux here?

They are very close, but there is a nuance. Craintif (fearful, timid, apprehensive) describes a state of being cautious or easily scared, which perfectly fits a traumatized animal. Peureux also means scared, but when applied to humans it can carry a negative tone like "cowardly." For a dog, both work, but craintif sounds more like a lingering psychological state.

Is starting a sentence with Que like this common in everyday spoken French?