What is the adjective (na't)?

A noun is named, then described. The description is the نَعْت; the noun described is the مَنْعُوت (man'ūt).

جَاءَ رَجُلٌ مُجْتَهِدٌ (jā'a rajulun mujtahidun) — a hard-working man came. Unlike English, the description comes after the noun.

A follower has no i'rab of its own. It copies the word before it — which is why the followers sit outside the twenty-two roles.

It copies its man'ūt in four things at once: i'rab, definiteness, number, gender. Change any one on the noun and the adjective changes with it — الرَّجُلُ الْمُجْتَهِدُ، الرَّجُلَانِ الْمُجْتَهِدَانِ، الرِّجَالُ الْمُجْتَهِدُونَ.

There are two kinds. A نَعْت حَقِيقِيّ describes the man'ūt itself. A نَعْت سَبَبِيّ describes something belonging to it — جَاءَ رَجُلٌ مُجْتَهِدٌ أَبُوهُ, a man came whose father is hard-working.

Do not confuse the نَعْت with the حَال.

A نَعْت describes a standing quality of the noun, and copies it in everything — definiteness included: جَاءَ الرَّجُلُ الطَّوِيلُ.

A حَال describes a passing state at the time of the action, is indefinite while its owner is definite, and is always manṣūb: جَاءَ الرَّجُلُ رَاكِبًا.

Definiteness is the quick test. Definite noun + definite describer = na't. Definite noun + indefinite describer in nasab = ḥāl.