Learn Arabic (Modern Standard): هذا الكتاب عبارة عن مجموعة من القصص القصيرة. - This book is a collection of short stories.

The phrase عبارة عن translates literally to "an expression of," but it is used idiomatically to mean "consists of," "is made up of," or simply "is." It is a very common way in Arabic to define or describe what something is, acting almost like an equals sign between هذا الكتاب and مجموعة من القصص.

Why is the adjective القصيرة in the singular feminine form when القصص means stories (plural)?

In Arabic grammar, plural nouns that refer to non-human things are treated as feminine singular. Since القصص (stories) is a non-human plural, its matching adjective القصيرة (short) must take the feminine singular form.

Why do we use من after مجموعة instead of making it an Idafa (possessive construction) like مجموعة القصص?

Both ways are actually correct, but using مجموعة من (a collection of) is much more common and sounds more natural in modern Arabic when referring to a general assortment. If you used the Idafa construction مجموعة القصص, it would sound slightly more specific, like "the collection of the stories" rather than "a collection of stories."

What is the singular form of القصص, and how is its plural formed?

The singular is قصة (story). The plural قصص is a broken plural that is formed by dropping the taa marbuta and slightly shifting the vowel sounds.