If you "can't get a word in edgeways", this means that you have no chance to say anything.
- So many people were trying to speak that I couldn't get a word in edgeways.
It's also possible to say "can't get a word in edgewise".
- Kate never stops talking. It's difficult to get a word in edgewise.
If you have forgotten what you were talking about, you have "lost your train of thought".
- Sorry, I've lost my train of thought after that interruption. What was I saying?
If you have absolutely no idea about something, you "haven't a clue".
- I haven't a clue what the capital of Kazakhstan is. Can you help me?
If you cannot understand something, you "can't make head nor tail" of it.
- I'm trying to assemble this IKEA table and I can't make head nor tail of the instructions.
If you forgot about something, it "slipped your mind".
- I was going to send it to you but I'm afraid it completely slipped my mind.
If you have to choose between two alternatives and you are finding it difficult, you are "caught between two stools".
- I'm not sure if I should fly to London or take the train. I'm really caught between two stools on this one.
If something tries to be two things but is not very good at being either it "falls between two stools".
- It's supposed to be a telephone and an MP3 player but it falls between two stools and isn't very good at either.
If you are trying to remember something and cannot quite remember it, it is "on the tip of your tongue".
- What's her name again? It's on the tip of my tongue.
If you don't understand something, you can say that "it beats me".
- It beats me why this car won't start. Everything appears OK with it.
Sometimes, explanations are too difficult or technical for us – they "go over our heads".
- He tried to explain the problem with the reactor but I'm afraid it just went over my head.
Sometimes we think and think about something, trying to remember a name, for example. You can say you have been "racking my brain".
- I've been racking my brain trying to remember the name of that lawyer we met last year in Dubai.