As I mentioned in my article “Beware of Fake Amazons,” the verb しまう is a pretty interesting one.

Adding it to the end of a て-form verb gives the feeling of having “completely” or “totally” done whatever the previous verb suggests.  Note that the main verb always takes the て form and that tense is expressed by しまう・しまった.  For instance:

  • 酔ってしまう [よってしまう] – to get completely drunk
  • 酔ってしまった [よってしまった] – “I got shitfaced”

しまう is more likely to be heard in “proper” conversation.  However, if you’re listening to people speak informally, you may be more likely to hear てしまう spoken as ちゃう.  This is because it rolls off the tongue nicely (and is, perhaps, a bit cooler-sounding).  Using the previous drunken examples:

  • 酔っちゃう [よっちゃう] – to get stinking drunk
  • 酔っちゃった [よっちゃった] – “I got totally hammered”

Do you see how the て+しまう morphs into ちゃう?  Notice also that the in 酔て, which doubles the sound of the next consonant (i.e. turning -te into -tte), remains unaffected.

Just remember:  て+しまう = ちゃう and て+しまった = ちゃった, and you should be fine.