The Complex しまう

しまう is a special verb that, when used with another verb in て form, can express one of two meanings. The first is "to carry out [that verb] with determination. It typically involves bringing something to a culmination point. You, in other words, do something completely, or finish doing something, or have something done."

The second meaning is "lack of premeditation or control over how things turn out. This often comes with the sense of regret; something regrettable happens, or you do something which you did not intend to."

Those two meanings (quoted from Genki) are quite different, and context is key in determining which is being used. I'm also going to use Genki's example sentences here since I'm less familiar with this grammar point.

本を読んでしまいました。
I read the book completely. / I finished reading the book.

As shown above, しまう is conjugated for tense, politeness, and all that good stuff. Note that the sentence could also convey something like "I (regrettably) read the book" if context dictated that you didn't actually want to read it.

電車の中にかばんを忘れてしまいました。
I inadvertently left my bag on the train.

宿題を忘れたので、先生は怒ってしまいました。
To my horror and sorrow, my professor got angry, because I had forgotten my homework.

In casual speech, ～てしまう and ～でしまう are often contracted to ～ちゃう and ～じゃう, respectively. They're conjugated as normal godan verbs.

新しい時計をなくしちゃった。
Crap, I've lost my new watch.

わっ、その トワイライト本を読んじゃった。
Ugh, I read that Twilight book.

ああ、プレゼントを忘れちゃった。
Oh no, I forgot your present.

Note that since the affirmative て form is used here, しまう can't be used to express regret over something that you didn't do. It's only used for regrettable things that will or have occurred.

I was reading over Tae Kim's grammar guide on this point to better solidify my understanding, and he stated that it's more common to use the "regrettable" meaning of  しまう. The "do something completely" meaning is only seen occasionally. So that's good to know.

I'm extremely happy to finally learn about this word (and its slang form), because it seems to be rather common in daily speech. I often saw ちゃった conjugations and could not find any formal conjugation charts that included such a thing, but now it makes sense.