Numbers from three to ten

From three to ten, the number takes the opposite gender of the singular noun: ثلاثة كتب, because كتاب is masculine, and ثلاث سيارات, because سيارة is feminine.

The counted noun is plural after three to ten, but singular and accusative from eleven on: عشرة طلاب, عشرون طالبا. One and two agree normally and come after the noun: كتاب واحد, بنتان اثنتان.

Eight shows the rule clearly: ثمانية أيام, but ثماني ساعات. Ten is عشرة before a masculine noun and عشر before a feminine one: عشرة رجال, عشر نساء.