That's a very good question indeed :) And yes , there are some rules on it : 1- After number 3 to number 10 , you write the name on the opposite of the number , masculine or feminine , like : عندي تسعة أقلام ثلاث تفاحات 2- After numbers 11 and 12 , you put the name as the same as the numbers , masculine or feminine , ( in the two parts ) , like : عندي أحد عشر كتابا عندي إحدى عشرة تفاحة عندي اثتا عشر بيتا عندي اثنا عشرة تفاحة 3- From number 13 , to number 19 , you put the first part of the number on the opposite of the name , masculine or feminine , and you put the second part of the number on the same of the name , masculine or feminine , like : عندي ثلاث عشرة تفاحة عندي ثلاثة عشر قلما عندي تسعة عشر بيتا عندي تسع عشرة ورقة -------------- Is that clear for you ? or you want more explanation ? :)

(3-10) the gender of the number will be the opposite to what is being counted. ثلاثة كُتب، أربع طالبات، أربعة طُلاب، خمسة أقلام، ثلاث بنات (1,2) follow the gender of what is being counted بنت واحدة، ولد واحد، بنتان اثنتان، ولدان اثنان

i don't think so but we usually say if we have 2 men (رجلان) and not (اتنان رجال) the rule of number two is to put the word like (فعلان) exemples : (two apples= تفاحتان.) (two tables= طاولتان) about the number three we can say both expressions (ثلاث بنات ) (ثلاثة بنات) both are corrects and : و الله اعلم