where did it come from?

on my quest to understand this piece of arabic grammar, my teacher asked...what does 'nahu' (arabic grammar) teaches us?

i thought n replied based on what i wanted from that lesson
-to teach me where to put each word in a correct place in a sentence!...so damn confident!...LOL

my teacher smiled...n shook his head...u got it wrong!

uhuh...?so, what do i learn from the subject nahu?...

thus, my teacher told me the story of how the word 'nahu' came from...

it was in the era of khalifah Ali bin Abu Talib...an incident happened to his friend, Abu Aswad adDu'ali...
Abu Aswad here came from a pure arabic descendant, and that day, he was sitting outside at night in the desert with his 18 year old daughter...they were looking up at the sky~

his daughter said "maa ajmalu asSamaa'?"...(what is the most beautiful thing about the sky?)
"nujumuha!"...replied Abu Aswad...(its stars!)
"la, lam uridu as-su'ala, wa innama aradtu at-ta'ajjub!"...she corrected...(no, i didn't mean it as a question, i meant it as an amazement!)
he was so shocked hearing his daughter saying that..."iftahii famaki!"...(open your mouth!)
[maa ajmalaa asSamaa'!]

worried on how his daughter who was also a pure arabic could make this huge mistake...Abu Aswad told Ali bin Abu Talib about their conversation that night...

'ya amir al-mukminin! last night i spent my time outside with my daughter.'...Abu Aswad started the conversation
'great!, you are a great dad!'...he replied
'we were looking up at the sky, and my daughter said..."maa ajmalu asSamaa'?"'
'tell her its stars!"
'yup, i told her that!...but she replied..."la, lam uridu as su'ala, wa innama aradtu at ta'ajjub!"'
'yaah!!!how can an arab made such mistake???' Ali was suprised!

worried that arabic language would be broken, Ali ordered Abu Aswad to write down that arabic words has three types..."al-ism, al-fe'l, al-harf"...and he added "unhu nahwan haza nahwi" (follow this path!)

and from there came the word 'nahu'...

but still,
my question has not yet been answered...what does 'nahu' teaches me?

my teacher replied politely...
"hua ilmun ya'rifu bihi dobtu awakhiri al-kalimati bi'anwaa'i al-harakati"
(it is the knowledge that teaches you with precision about types of line at the end of each word)

only???

hurm...
to learn whether you used the correct word for a meaningful sentence we study 'balaghah'
and so on...
owh why dun i like history before???...LOL