Grammar: Use of numbers

Numbers in Arabic are quite complicated, there are different rules for the numbers, numbers are declined according to gender. Getting the grip on numbers in order to make practical use of them (few Arabs used numbers correctly), is however reasonably easy.
From 21 to 99 you count like this: (example) 24: Four wa-twenty.From 12 to 19 you count like this (example) 15: Five Ten. 11 is slightly slightly diverging.
When putting numbers together with nouns you do like this:

• 1: (example) 1 book is said as simply as "book", "kit?b", you leave 1 out, unless it is very important to emphasise that it is one book.
• 2: (example) 2 books is a special case, as Arabic not only has singular and plural, but also dual. The rules here are straight, but often ommitted by students, who wind up saying "2 books",
  ithn?n kutub.
  That is not correct, and the correct dual for 2 books is
  kit?b?ni.
  
• 3 and up: You place the full form of the number first, immediately followed by the noun: 42 books:
  ithn?n wa-'arba^c?n kutub.
  While this is not the correct form, it is OK to say it this way at the present level. If you're curious, this is the correct way for saying 42 books:
  ithn?n wa-'arba^c?n kit?b?n.

0	—	sifr		1	—	wāhid
2	—	ithnān		3	—	thalātha
4	—	'arbaca		5	—	khamsa
6	—	sitta		7	—	sabca
8	—	thamāniya		9	—	tisca
10	—	cashra		11	—	'ahada cashra
12	—	ithnā cashra		13	—	thalātha cashra
14	—	'arbaca cashra		15	—	khamsa cashra
16	—	sitta cashra		17	—	sabca cashra
18	—	thamāniya cashra		19	—	tisca cashra
20	—	cishrūn		21	—	'ahad wa-cishrūn