If you can't understand something, you can say that you can't make heads or tails of it.

If you memorize something, you learn it by heart.

If you are talking about something, and then you forget what you are saying, you lose your train of thought.

If something is meaningful to you or has value, you get something out of it.

If something is memorable and you can't forget it, it sticks in your mind.

If you come to grips with something, it means you finally understand it.

If you can't get your head around something, it's too difficult to understand.

If you say the first thing you think about, you say it off the top of your head.

If you don't see the point of something, you don't think it's worthwhile.

If something is beyond you, it's either too hard for you, or it's something that you simply can't understand.

If nothing comes to mind, you can't think of an example or idea.