Dear Hafsah,
As-Sabru Jameel, means "Patience is beautiful", Patience there having the standing of "The Patience" as a trait. Sabrun Jameelun means "beautiful patience", which is linguistically referring to having patience in hardship, unidentified patience that is. As-Sabru Jameel is actually an expression describing or reminding of having beautiful patience, while Sabrun as it stands is the invitation to maintain beautiful patience. The second is mentioned in the Quran:
فَاصْبِرْ صَبْرًا جَمِيلًا "Fa
isbir
sabran jameel
an":Therefore do thou hold Patience,- beautiful patience (in contentment).[70:5]
Also Jacob -a.s.- says it in the Quran when his sons lost Joseph and when years later they came back without Joseph's brother:
وَجَآؤُوا عَلَى قَمِيصِهِ بِدَمٍ كَذِبٍ قَالَ بَلْ سَوَّلَتْ لَكُمْ أَنفُسُكُمْ أَمْرًا
فَصَبْرٌ جَمِيلٌ وَاللّهُ الْمُسْتَعَانُ عَلَى مَا تَصِفُونَ
Wajaoo AAala qameesihi bidamin kathibin qala bal sawwalat lakum anfusukum amran fasabrun jameelun waAllahu almustaAAanu AAala ma tasifoona:
"and they produced his tunic with false blood upon it. [But Jacob] exclaimed: "Nay, but it is your [own] minds that have made [so terrible] a happening seem a matter of little account to you! But [as for myself,] patience in adversity is beautiful; and it is to God [alone] that I pray to give me strength to bear the misfortune which you have described to me."
Similarly, Al-Ilmu nurun is "Knowledge is enlightenment", refering to having Knowledge as a trait. It is identified. There is no expression that says Ilmun nurun in the unidentified like that. There is however a duaa: ربي زدني علما "Rabbi zidnee ilman": God, grant me (some) knowledge.
So to answer the questions:
- As-Sabru Jameel does indeed mean "patience is beautiful"
- People say Sabrun Jameelun because it means (maintain) beautiful patience, and because it is the one used in the Quran.
- You can indeed if you wish for example say أي علم نور "Ayyo Ilmin nurun" which means "Any knowledge is light"
- kitabun ahmar works because ahmar is an adjective describing the book, but Al-Ilmu nurun is more "Knowledge is (like) light", nurun is not an adjective. So if you say Ayyo Ilmin nurun, you're really saying "Any knowledge is (like) light". And nur in arabic only means the light that shines, it does not mean the adjective opposite to heavy or opposite to deep, for those are actually خفيف "khafif" and فاتح "fateHH" respectively.
Hope that helped remove a bit of confusion (instead of adding :))