Phrase(s): I don’t understand (it). [and] I can’t understand (it).
I am confused and bewildered (by what has happened). • Bill: Everyone is leaving the party. Mary: I don’t understand. It’s still so early. • Bob: The very idea, Sue and Tom doing something like that! Alice: It’s very strange. I can’t understand it.
McGrawhill's American Idioms And Phrasal Verbs