Here’s one Japanese grammatical form that I rarely find discussed. The following scene is from Ueshiba Reach’s Discommunication , v.2:

Reading right to left:

“What are you doing?!”

“Each and every one of these drawers has a precious treasure of mine! Don’t just go about messing with them!”

“Treasure?”

“Ah! Now’s not the time for this!!” “Busy, busy!”

“…”

Akechao… (Let’s just open it…)

[clunk]

A while later:

[slip]

“Now how about this other drawer? Akechao”

“Hm?”

[stare]

This is the abbreviated -ō inflection of -chau, itself the abbreviation of -te shimau. If you listen to the spoken language at all, or if you ever read any manga, you already know -chau. When I first started learning Japanese, I recall how much trouble this form gave me; it wasn’t found in my textbooks, nor grammar books, nor dictionaries. (Similarly, a narrator from Helen DeWitt’s wonderful The Last Samurai rejoices at finding a then-unusual little book on colloquial Japanese abbreviations, which finally solves the problems that had plagued him since forever.) Those were the dark ages of Japanese education, kids. They taught us starting from -masu forms, they told us to use oru and de gozaimasu, they told us cursive kanji were “necessary for daily life in modern Japan”, and they thought it improper to teach “wrong” Japanese. Luckily, these days people have learned the value of teaching a language as she is spoke, and today no one would have any trouble finding -chau in a dictionary or textbook.

Of course, if I knew how to search adult grammars, I’d have found it even in the past century. Martin’s godlike Reference Grammar (1975) thoroughly details all possible combinations of -te shimau with -wa, -nai, causatives/passives, etc., all possible contractions including dialectal differences, and illustrates no less than five meanings:

Finishing an action (perfective aspect): Tsui ni Taiyō ga shizunde shimatta “Finally the sun finished sinking”. Doing something all the way through, completely (completive aspect): O-Kane wo otoshite shimatta “I lost all the money”. Ends up doing, gets around to doing; Tabe-sugite o-Naka wo kowashite shimatta “Being such a glutton, I ended up with a ruined stomach”. Just a strong or emphatic past: Ichatta “They’re gone!” As a mood indicator, it marks annoyance, displeasure at how things ended up, frustration of expectations: Nan de mo nonjau “He’ll drink any damn thing!”.

For didactic purposes we could perhaps classify those, grosso modo, in two main strands of meaning: to finish doing (completive aspect) and to end up doing despite one’s will (non-volitional mood). As the English translations (“finish”, “end up”) suggest, both have to do with the meaning of shimau as an independent verb: “to finish, to store away”.

This is why I like -chao (-te shimaō) so much. -ō denotes volition; that is, something she wants to do. -chau denotes ending up doing something, despite of oneself—that is, involuntarily (kowachatta! “I accidentally broke it!”). The combined volition-nonvolition effect is resembles “I baked you a cookie, but I ate it”: “let’s end up opening the drawer!” “let’s put ourselves in the state of ‘whoops, I’ve opened it!’”“let’s just open it [and not worry about the consequences]!”