The definite article
Arabic · Key Stage 2 · The School
A book or the book
English marks the difference with two separate words, a and the. Arabic marks it on the noun itself: attach the article and the noun is DEFINITE — the book, a particular one. Leave it off and the noun is INDEFINITE — a book, any book. The difference matters constantly, because Arabic sentences are built out of it.
Tanween marks the indefinite
An indefinite noun usually carries TANWEEN, a doubled vowel mark at its end that adds an -n sound: kitab becomes kitabun. So the ending itself tells you the noun is indefinite before you have read anything else. Tanween is written above or below the final letter and is a grammatical signal, not a decoration.
Never both at once
A noun takes the article OR tanween, never both, because they mark opposite things — one says definite, the other says indefinite. Seeing both together is a reliable sign of an error. This single rule catches a great many mistakes in early writing and is worth checking every time you write a noun.
بَيْتٌ — a house, indefinite, and the ending is tanwīn (the -un sound, written as a doubled ḍamma). البَيْتُ — the house, definite, with ال and a single ḍamma. Notice you can have ONE or the OTHER but never both: الْبَيْتٌ is impossible, because ال and tanwīn are two ways of answering the same question. Spotting that rules out a great many misreadings.