What is the subject (mubtada')?
In one line
Classical definition
Understand it
A جُمْلَة اِسْمِيَّة (jumlah ismiyyah) opens with an ism, and that opening ism is the مُبْتَدَأ. It is what the sentence is about. What is then said about it is the خَبَر.
In مُحَمَّدٌ رَسُولٌ (muhammadun rasulun), Muhammad is a messenger, مُحَمَّدٌ is the mubtada' and رَسُولٌ is the khabar. Both carry rafa. Both must.
The two parts of a nominal sentence
The mubtada' is what you are talking about; the khabar is what you say about it. Both are marfu'.
Notice what Arabic does not need. English cannot say “Muhammad a messenger” — it must insert is. Arabic completes the sentence in two words, with no verb at all, and the two dammas hold it together.
The mubtada' is defined by what stands before it: nothing. The moment a governing word such as إِنَّ or كَانَ enters, the word is no longer called a mubtada' — it becomes the ism of that governor, and may no longer be in rafa.
How to spot it
In the Qur'an
In the language — فِي اللُّغَة
The word مُبْتَدَأ (mubtada') in everyday Arabic means a thing begun, a starting point.
اِبْتِدَاءُ الْعَامِ (ibtida'u al-'ami) — “the beginning of the year”
As a grammatical term it means the ism a nominal sentence begins with. Do not confuse it with مُبْتَدِئ (mubtadi'), which means a beginner.
Related terms
Common questions
Can a mubtada' be indefinite?
Rarely, and only with a reason — a preceding negation or question, a description attached to it, or a khabar that has moved in front of it. Left bare and indefinite at the head of a sentence, it does not work.
Next in the guideاَلْخَبَرWhat is the predicate (khabar)? →Updated