Where I would expect a long ì on "a thidseir", there isn't one. Is there a helpful rule?
Yes im struggling when to use the a ir not. Keep getting it wrong
Are you asking about the difference between tidsear and a thidseir? That's the vocative case, used when directly addressing someone. Tidsear on its own is in the nominative case.
I don't think you ever would. :)