These are correct: If anybody /somebody finishes writing his/her test earlier , s/he can go home. Have you ever been on a plane? -- - In is possible but unusual.
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Mister Micawber.Once you've finished writing the test you can go home earlier. ---how about this one? is the present perfect tense OK here or not?
It sound unimaginably awkward at best. Would you care to put it into your sentence so I can see what you are thinking?