What exactly is the difference between <verb>-てしまう and <verb>- [切]{き}る?


I've read that both the ～てしまう and ～きる (18th meaning of 切る at
<http://www.csse.monash.edu.au/~jwb/cgi-
bin/wwwjdic.cgi?1MUE%E5%88%87%E3%82%8B>) forms are used to signify something
has been finished/done/completed.

If so, what exactly is the difference between the two conjugations (how do we
decide when to use which)?

example:

1) もう荷造りしてしまった。

2) もう荷造りしきった。




First, the ～てしまう construction can convey a sense of regret, which the 切る verb
suffix cannot:

> 花瓶を落として割ってしまった。 I dropped the vase and [regrettably] broke it.
>
> 花瓶を落として割り切った。 (unnatural)

When used in constructions expressing the completion or finishing of an
action, 切る tends to sound best with actions that can be measured on a scale,
but there's a lot of overlap:

> 使い切る use up (使ってしまう could work)
>
> 乗り切る completely overcome (乗ってしまう sounds unnatural)
>
> 冷え切る cool off entirely (冷えてしまう could work)
>
> 疲れ切る become exhausted (疲れてしまう could work)
>
> 出し切る put forth everything (出してしまう might work in some cases)

All of the verbs above imply the exhaustion of a finite quantity (使う, 疲れる, 出す)
or a change that could stop at any point on an invisible scale (乗る, 冷える). This
aspect is reflected in the fact that the generic verb for "run out" is 切れる:

> ノートパソコンの電池が切れた。 My laptop's battery died.
>
> おい、この納豆の賞味期限が切れているよ。 Hey, this nattō's past its expiration date, you know.

On the other hand, for actions that don't fit into a nice scale (i.e. they
either happen or they don't), the ～てしまう form is usually better:

> 泣き出してしまう burst into tears (泣き出し切る sounds unnatural)
>
> 変わってしまう change completely (変わり切る might work in rare cases)
>
> ほれてしまう fall head-over-heels in love (ほれ切る sounds unnatural)
>
> 消えてしまう disappear entirely (消え切る might work in rare cases)




First, the ～てしまう construction can convey a sense of regret, which the 切る verb
suffix cannot:

> 花瓶を落として割ってしまった。 I dropped the vase and [regrettably] broke it.
>
> 花瓶を落として割り切った。 (unnatural)

When used in constructions expressing the completion or finishing of an
action, 切る tends to sound best with actions that can be measured on a scale,
but there's a lot of overlap:

> 使い切る use up (使ってしまう could work)
>
> 乗り切る completely overcome (乗ってしまう sounds unnatural)
>
> 冷え切る cool off entirely (冷えてしまう could work)
>
> 疲れ切る become exhausted (疲れてしまう could work)
>
> 出し切る put forth everything (出してしまう might work in some cases)

All of the verbs above imply the exhaustion of a finite quantity (使う, 疲れる, 出す)
or a change that could stop at any point on an invisible scale (乗る, 冷える). This
aspect is reflected in the fact that the generic verb for "run out" is 切れる:

> ノートパソコンの電池が切れた。 My laptop's battery died.
>
> おい、この納豆の賞味期限が切れているよ。 Hey, this nattō's past its expiration date, you know.

On the other hand, for actions that don't fit into a nice scale (i.e. they
either happen or they don't), the ～てしまう form is usually better:

> 泣き出してしまう burst into tears (泣き出し切る sounds unnatural)
>
> 変わってしまう change completely (変わり切る might work in rare cases)
>
> ほれてしまう fall head-over-heels in love (ほれ切る sounds unnatural)
>
> 消えてしまう disappear entirely (消え切る might work in rare cases)

