"Alasdair is understanding."
Translation:Tha Alasdair a' tuigsinn.
I suppose that, as all the other verbs are in the present continuous, this one can be used in Gaelic in this tense as well. However, this is not the case in English. Of course, your suggestion also works in English, but there understanding is an adjective. I think the problem is that to nderstand is not used in the continuous form in English but can be used so in Gaelic.