1) 小さい vs 小さな Among your examples, [b)chiisai no kuruma] is not correct. You can use both [a)chiisai kuruma] and [c)chiisana kuruma]. The meanings are almost same. There is just a slight difference. I feel that chiisai is more objective, and chiisana include some emotional affection. (Is this something similar to the difference between "el carro pequeña" and "el pequeña carro"?) 2) Kirei(きれい) This is not an -i adjective. "kirei" is a chinese origin word, and it happens to end with 'i'. "kirei-na" becomes -na adjective, but "kirei" itself is a noun. So, you can't say 'kirei hana', 'kirei no hana', 'kire na hana'. These are all incorrect. You should say 'kirei-na hana'. 3) 'wasurechaitenai' may be 'wasurechattenai?'. 'wasure'+'chatte(te-simatte)'+'nai?' (Have you all forgotten, haven't you?)