Learn French: Que l'abandon des chiens soit si commun est vraiment triste. - That the abandonment of dogs is so common is really sad.

When a sentence begins with a Que clause acting as the subject, the verb inside that clause usually takes the subjunctive. This is strictly required here because the main clause (est vraiment triste) expresses an emotion or subjective judgment.

Could I just say Il est vraiment triste que... instead of starting with Que?

Yes, Il est vraiment triste que l'abandon des chiens soit si commun is perfectly natural and actually more common in spoken French. Starting the sentence with Que is a bit more formal and puts strong emphasis on the fact itself.

Why is the main verb est singular when the sentence mentions multiple dogs (chiens)?

The subject of the verb est is not the dogs, but rather the entire clause Que l'abandon des chiens soit si commun. Whenever a full clause acts as the subject of a sentence, the main verb must take the third-person singular form.

Why does it say des chiens instead of just de chiens?

In French, des here is the contraction of de (of) and les (the). While English says 'abandonment of dogs' without the article, French requires the definite article les when referring to a noun in a general sense.