"I am not understanding Donald."
Translation:Chan eil mi a' tuigsinn Dòmhnall.
I think that "I am not understanding Donald" can be interpreted correctly both as "Chan eil mi a tuigsinn a Dhòmhnaill" (i.e. vocative) and "Chan eil mi a tuigsinn Dòmhnall" (i.e. nominative). To accurately distinguish between the two options would require some more information on the circumstances. I realised that as I typing but was forced to make a random guess ... unfortunately I plumped for vocative!
a' is short for ag which changes understand to understanding. Verbatim translation of Tha mi a tuigsinn...: "am. I understanding.... This makes the act of understanding actively happening currently. I like to think of Scots as proactive, men of action when it comes to their verbs!