You are still free to study how an AGPL program works and adapt it to your needs. However, you just have to pass the same freedom to the users of your modified programs which means the source code must be available to them.

Also, if some of the adaptations you've made are liked by the original developer, he can integrate them into the main version. So both of you mutually benefit from each others work, rather than one just leeching on the other.