when you count (just say 1,2,3...etc) you use masculine. but when you say the number of the things (ex:one girl) you should use feminine with feminine things and vice verse. ex: كتاب واحد One book *(كتابان (اثنان Two books بنت واحدة One girl *(بنتان (اثنتان Two girls *It's not important to say "Two" like in my examples,so I put them between brackets. Hope that helps:)

I want to add just that there is a rule in Arabic to determine (masculine or feminine) in the sentence according to the counted noun example; I saw two girls girls in Arabic is a feminine noun in its singular form so the number 2 should be feminine the rule says if the counted noun are 1,2or 3 [2 girls-1 boy-3 cars] so the numbers 1,2 or 3 should match the counted noun in its gender masculine noun =masculine number طفل واحد feminine noun =feminine number شجرتان اثنتان example I saw two girls= noun is 2 and its singular form is feminine so number must be feminine رأيت فتاتان( اثنتان I saw two boys=noun is 2 and masculine so number must be masculine رأيت صبيان(اثنان while if the noun's number 4,5,6,7,8,9 so the number must be in opposite gender example; I saw five men= noun are and 5 masculine so the number its opposite= feminine رأيت خمسة رجال خمسة=مؤنث feminine رجال ----رجل=مذكر masculine I saw five girls رأيت خمس بنات masculine number= خمس=مذكر بنات----بنت=مؤنث=feminine counted noun the numbers [10,20......till 100 ] is always same no gender for them as same as [1000,million,billion...] I'm sorry if I couldn't explain in a good way while you are welcome if you need any further explanation or question

واحد is used for male person or for number 1 واحدةis used for female person more question please add me on skype zaid.aldeneh