Complex sentence: When Ben wakes up to realizes that his powers are gone, he tries to warn Reed Richards that the Thing that's with him is an impostor, however they believe that Ben himself is the impostor and dismiss him from the Baxter Building.
Split into simpler sentences: When Ben wakes up to realizes that his powers are gone, he tries to warn Reed Richards that the "Thing" who is with him is an impostor. However, they believe that Ben himself is the impostor, and dismiss him from the Baxter Building.