There aren't exactly much of things that Kurt doesn't understand.

He can read, write (a bit but he prefers reading anyway), ride a bike and he knows what the word mischievous means (his mom never had to tell him what it meant even though it's a long word). He can be quiet when he's told to, he understands a lot of fashion even though he's only seven and when the teacher asks the class who can sing this song or that song, he's more often than not the only one to lift his hand, even if it's a girls' song.
He understands that pink is a girly color and that people on his class can be mean if he wears pink, he learns that if you upset certain people on the class they will do something such as toss you into dumpsters or tackle you when no-one sees or steal your homework.

What he doesn't understand, though, is why his teacher wants to have a talk with his parents after they return their stories to the teacher after the English class. His feet get numb and he fiddles with the buttons of his coat, the clock is almost half past three already and he's getting impatient, the shades of his parents are almost-visible through the weird glass of the door and he can hear his fathers raised voice and his mother shushing him, and he frowns. He hopes his story wasn't that bad.
His parents come out soon enough, and even though there's not much he doesn't understand, he doesn't understand why his mothers eyes are a little red and her face is wet and why his father is huffing in anger.

He doesn't exactly get why they suddenly want to buy him ice cream and watch Disney movies with him when they know he has homework and they never do this on Wednesdays.

He doesn't get why his mother tells him that she will love him no matter what, doesn't understand why she kisses him two times instead of one and on the cheek instead on the forehead, but it's nice so he doesn't say anything, just goodnight and I love you too.

But most of all, he doesn't understand why two princes can't get married.