I can't make it out.
I don't understand that.
I don't understand that at all.
I cannot comprehend that at all.
I do not understand it at all.
The matter is above my apprehension.
I don't really understand that.
I hardly understand that.
I don't understand that meaning.
I hardly understand that.
I cannot understand.
I haven't understood that very well.
I couldn't really understand that.
My understanding of that is inadequate.
I can't really understand that.
I don't really understand that.
I couldn't understand that.