Polite, Conversation & Urban (slang) English
Subject: "Forgetting things"
Formal English (extra polite):
1. Oh dear, I'm afraid I can't remember.
2. Oh so sorry, I've completely forgotten.
3. Sorry, I have no recollection of...
1. My mind's gone blank
2. Sorry. I have no memory of...
3. I'm afraid it doesn't ring a bell.
4. Sorry, I just plain forgot.
5. I simply forgot to do it.
6. What was I thinking of?
7. Oh no, it completely slipped my mind.
"Urban" English (street slang):
1. I had a total brain freeze.
2. I must be getting alzheimer's!