When we want to ask the question, who are you writing to?’ we use cò (who, which) + thuige (gu + e), the masculine singular form.
Cò thuige a tha thu a’ sgrìobhadh?
To whom are you writing?
We can use cò (who, which) with other prepositions as well. You’re already very familiar with Cò às a tha thu? Cò is always followed directly by the prepositional pronoun (third person masculine form).
Cò ris a tha thu a’ bhruidhinn?
To whom are you speaking (who are you speaking to?)
Tha mi a’ bruidhinn ri Peadar.
I am speaking to Peter.
Cò leis a tha an leabhar sin?
Whose is that book?
‘S ann le Fearchar a tha e.
It’s Farquhar’s book.
Cò aige a tha fios?!
Chan eil fios aig duine!
Cò ris a tha an t-sìde coltach?
What is the weather like?
Tha i fliuch is fuar!
It’s cold and wet!