Final answer:
The sentence 'don’t do it' is the one not in past tense, as it is an command in the present tense, while the rest of the sentences are in past tense.
Step-by-step explanation:
In answering your question, the sentence that is not in past tense is: 'don’t do it'. The rest of the sentences, 'it’s been done', 'who did it?', and 'oh no, he didn’t!', are indeed in the past tense.
Past tense is used to talk about a completed action in a time period before now. 'Don’t do it' is an example of the imperative mode in present tense, which gives a command, advice, or makes a request.
Learn more about Tenses