The nominal sentence: two words, one statement
Arabic · Key Stage 2 · The School
Subject and predicate
A nominal sentence has two parts: the MUBTADA (what you are talking about) and the KHABAR (what you say about it). البَيْتُ كَبيرٌ — the house is big. No word for "is" appears; the two parts standing together IS the statement. Arabic is more economical than English here.
The definite article does the work
Notice: البَيْتُ (the house, definite) كَبيرٌ (big, indefinite). That pattern — definite then indefinite — is what makes it a sentence. If both were definite (البَيْتُ الكَبيرُ), you would have a PHRASE: "the big house", not a statement. One article changes a description into a claim.
Building your own
Try: الوَلَدُ صَغيرٌ (the boy is small), المُعَلِّمُ جَديدٌ (the teacher is new), الكِتابُ مُفيدٌ (the book is useful). Write five of your own with words you know. This structure appears constantly in the Quran, so recognising it will start paying off in your reading immediately.
الكِتابُ الكَبيرُ — 'the big book'. Both words are definite, so this is a PHRASE; it names something and says nothing. Now drop the ال from the second word: الكِتابُ كَبيرٌ — 'the book is big'. That is a SENTENCE. Definite noun, then indefinite description, and no word for 'is' anywhere. The definiteness is what tells the reader where the statement begins.