What is the masculine (mudhakkar)?

Every Arabic ism is either masculine or feminine — there is no neuter “it”. Gender matters because everything that refers back to a noun — adjective, pronoun, verb, demonstrative — must match it, so one wrong guess ripples through the whole sentence. When unsure, use the pointer test from the definition: if هَذَا sits naturally before the word, treat it as masculine.

Masculine in Arabic grammar is not a claim about the world. It is the default category: a noun is masculine unless something marks it feminine.

That is why كِتَاب (a book) and بَيْت (a house) are masculine — not because anything about them is male, but because nothing marks them otherwise.

Are plurals masculine or feminine?

Rational beings keep their natural gender. Broken plurals of non-rational things take feminine SINGULAR agreement — الكُتُب agrees like “she”. This one rule explains a great deal of Quranic agreement that otherwise looks irregular.