Well, to form the comparative/superlative (اِسم التفضيل), some conditions must apply to the verb it's delivered from. If one of the conditions doesn't apply to it, you can't form اسم التفضيل from it directly. Instead, you use اسم التفضيل of a suitable verb that all conditions apply to + the verbal noun المصدر of the main verb.
These conditions include: (I'll just put the ones that will help you to form اسم التفضيل to not confuse you. Being aware of the others isn't necessary for you, but it can confuse you)
First: The verb must be ثلاثي (of three letters), like: كَبُرَ (the adjective from it is كبير),
صَغُرَ (the adjective from it is صَغير),
طَال (the adjective from it is طَويل)
Second: The adjective delivered from the verb mustn't be like these: أحمر، أصفر، أخضر، أعرج، أحول (that their feminine forms are حَمراء، صفراء، خضراء، عرجاء، حولاء)
For example: اسم التفضيل can't be delivered directly from this adjective: أبيَض, because it's like the ones above and its feminine is بيضاء. So, we use اسم التفضيل:
أكثَر
+ بَياضًا (which is المَصدَر)
هَذا الثوب أكثر بياضا من ذاك.
Examples for verbs that are used for this purpose (+ المصدر of the main verb): أكبر، أكثر، أحسن، أفضل، أجمل -------------------------------------------------------------------------------------------------------------
*You can use this way also when اسم التفضيل can be formed directly.
Examples: اسم التفضيل for طويل can either be أطول or أكثر طولاً.
اسم التفضيل for نحيف can either be أنحف or أكثر نحافةً.
Hope this helps.
If you still have questions, let me know.