"Calum is as fit as a deer."
Translation:Tha Calum cho fallain ri fiadh.
It depends on whether the name is used in a nominative case, as in the sentence above, or vocative case: Calum > Madainn mhath, a Chaluim.
Later (in the hopefully soon to be added newer part of the Gaelic tree) we'll also get to enjoy the dative case and the genitive case. So you'll have something to look forward to.