) actually it is simplier than it may appear. After you said a digit "один" at the end of number 1/21/101/1001 (один/двадцать один/сто один/тысяча один) you have to use singular Nom. But number 11 (одиннадцать) doesn't have "один" at the end. Like English eleven doesn't have "one" but "twenty one" and onwards have. The same thing with other numbers for example 2/22/102... (два/двадцать два/сто два) but "двенадцать" (analogically English "twelve") and so on. With "два"/"три"/"четыре" you should use Sing. Gen. If there is no word "один"/"два"/"три"/"четыре" at the end of a number you must use pl. genitive.