困难在于

困 = 囗 + 木 : New mnemonic symbol to avoid confusion with 夗: sleeping pills.

Karl Koala (ku) prepared a nest made out of Christmas trees (木) stuffed into a plastic enclosure (囗) in the encampment's bathroom ((e)n4). To make sure that he sleeps (困) really well he pops sleeping pills (困) before laying down in his nest.

难 = 又 + 隹 : Mnemonic symbol: a video game which is almost impossible to beat.

难 = 又 + 隹 : Just inside the anthill's kitchen, Napoleon and a sparrow are trying to beat an insanely difficult video game. After playing and failing all day long Napoleon finally loses his temper and crashes the game wearing two boxing gloves.

在 = 一 + 亻 + 土 : Zorro (z) and Rosa Luxemburg (亻) exist (在) in the airplane's bathroom (ai4). Rosa is in the middle of balancing a flute (一) on her head and on a pile of clay (土). Zorro uses his rapier to inscribe "Z" into the flute, so as to mark that the pronunciation of "在" begins with "z".

于 = 亅 + 二 : Mnemonic symbol: prepositions (in, at, on, to, ...) are what glue the sentence together. Honey is as sticky as glue, so I'll represent 于 by a honey jar.

Yu the Great (于) was assigned to repair the electronics of the space station's kitchen (Ø2), but instead he was snacking honey from a honey jar (于). Unfortunately his capacitor (二) got stuck in the honey and now he has to use a crowbar (亅) to lever it out.