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LingakuLingaku[verb て-form] + しまう [verb て-form 〜て → 〜ちゃう] (casual) [verb て-form 〜で → 〜じゃう] (casual)
てしまう follows a verb's て-form and has two meanings: the action is done and finished, or it happened when you didn't want it to. Often it is a bit of both. しまう changes like any う-verb: てしまった, てしまいます.
A verb whose て-form ends in で takes でしまう: よんでしまう読んでしまう, しんでしまう死んでしまう. The dictionary form never goes in front.
Most of the time てしまう says the result is a pity, an accident or a slip: losing, forgetting, breaking, saying too much. English has "ended up", "accidentally", or just a sigh.
With a verb for something that happens rather than something you do, it says the thing is gone and can't be brought back.
With ぜんぶ全部, もう or すっかり, てしまう says the action is finished and nothing is left. It works for the future too: you get the job out of the way.
In conversation てしまう shortens to ちゃう, and でしまう to じゃう. Both change like any う-verb: ちゃった, ちゃって, ちゃいます. With people above you, keep the full てしまいます.
じゃう goes only where the て-form ends in で.