My gf pretends to forget things i don’t understand why?

Take a moment and think about the things she pretends that she forgot.. What kind of things? Things that concern your relationship? Your family? Third people? Can you put all these things in one category ? If yes, start thinking about the specific things that may "trigger" this behaviour and then talk with her on this specific matter. If not, start thinking about other issues that may - indirectly - be linked to a deliberate behaviour of this kind from her side. What are the secondary benefits of this behaviour for example? Sometimes a behaviour is just the superficial effect of something else ;)

Are you very sure she's pretending? Many people have tunnel vision so to speak and have a hard time remembering details that pertain only to others and not to them. The other possibility is she maybe doesn't believe you don't understand and therefore doesn't acknowledge it. If you truly don't comprehend something, perhaps sit down and have a serious talk with her about it. It should solve the issue by letting her know you're being sincere and also help her commit it to memory. If she still is forgetting about it, then you need to have a very different kind of conversation about her problems.