Gender
Every noun is masculine or feminine, and every verb, adjective, and pronoun has to agree.
Arabic, like French or Spanish or German, assigns a gender to every noun. There are only two — masculine and feminine, no neuter — and the gender is not optional. It is part of the word. Verbs and adjectives must match it. For an English speaker this is the part of Arabic that requires the most sustained background attention, because English has nothing like it: we have natural gender on pronouns ("he," "she," "it"), but our nouns are bare, and our verbs and adjectives don't change for gender at all.
The basic rule
The clearest signal is the suffix ة — taa marbuuTa, "tied taa." Most nouns ending in -a written with this letter are feminine. Most nouns without it are masculine.
The same rule extends across the rest of the grammar. A feminine noun takes a feminine verb (with prefix ta-), a feminine demonstrative (haadhihi, "this"), a feminine pronoun (hiya, "she/it"), and so on.
The exceptions
Three groups of exceptions are worth knowing on day one.
Feminine nouns without ة. Many feminine nouns lack the visible suffix and have to be learned. Body parts that come in pairs are reliably feminine. So are most countries and a number of natural features:
Masculine nouns with ة. A small group of words ending in ة are masculine in spite of it. The classic example is khaliifa (caliph), which is grammatically masculine because the referent is. Other examples are ʿallaama ("great scholar," masculine, used as a title) and personal names like Hamza and Muʿaawiya. The ة here is a kind of intensifier rather than a feminine marker.
Pairs of opposites without parallel forms. Some male/female pairs are not built from one stem with a feminine suffix added; they are entirely separate words:
How gender forces agreement
Gender is not a label hanging on the noun in isolation; it propagates. A feminine subject changes the verb. The past-tense verb gets a -t suffix; the present-tense verb gets a ta- prefix.
Why English speakers struggle
Two failure modes are common. First, forgetting to agree. The default English habit is "noun is just a noun," so the brain happily produces al-bint kabiir ("the girl big" — masculine adjective). Native speakers register it the way you would register "the girl is bigs." Second, mis-gendering everyday non-ة feminines like shams, arD, yad, ʿayn, or country names. These have to be drilled, ideally with a memorable example phrase rather than a list.
One useful habit: when you learn a noun, learn it with a small adjective phrase ("a hot sun," shams Haarra) rather than alone. The adjective forces you to remember the gender.
What it's called in the Arabic tradition
Masculine is مُذَكَّر (mudhakkar), feminine is مُؤَنَّث (mu'annath). The taa marbuuTa itself is تاء مَرْبوطَة — literally "tied t" — written like a haa with two dots above it. When followed by a suffix or a possessive ending, the tie unties and the letter becomes a regular t sound: madiina becomes madiinatii ("my city"), revealing the t.