Why not include formal or plural n'oubliez?
I'm not sure why it isn't "n'oublies pas de t'enregistrer" or n'oubliez pas de vous enregistrer". Thank you
Your « vous » version is correct (& accepted); for your « tu » version, it should be « n'oublie » without the 's' because you drop it for the "tu" imperative for first group verbs (i.e. those ending with '-er' in the infinitive form)! :)
Don't "you" (implied) forget to check in. Tricky that.
More correctly "Don't forget to check yourself in." - it's reflexive! :)
why isn't formal accepted?
en avance du vol - as an alternative?
So 'enregister' is reflexive?
When it's used to mean "check (yourself) in" to a flight, yes! :)
Who checks in one day before a flight? No, you confirm your flight the day before and check in at the gate when called to do so by the airline attendants.