We can make ちゃいけない even more informal by using the word ダメ instead of いけない.
| You must not drink alcohol tonight. | 今夜 アルコール を 飲んじゃ ダメ だ。konya arukooru o nonja dame da. |
| You must not eat in bed. | ベッド で 食べちゃ ダメ。beddo de tabecha dame. |
When using ちゃダメ or じゃダメ, it's acceptable to either include or exclude the だ at the end of the sentence. Including the だ tends to be a bit more assertive or emphatic, while excluding it makes the sentence a bit softer or more informal.
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