In the present tense the long construction, involving 'tha' and the verbal noun, covers both the continuous and simple present i.e.
Tha mi a' sgrìobhadh - I am writing, I write
In the past tense the long construction generally only covers the continuous and not the simple past tense i.e.
Bha mi a' sgrìobhadh - I was writing
Sgrìobh mi - I wrote
Shuas sgrìobh mise 'dh'fhuirich mi ann an Glaschu'. This seems ok, however, fuireach alongside iarraidh, tuigsinn and smaoineachadh are exceptions to the rule. In the past tense, with the long construction, these work as if they were in the present tense (with both continuous and simple) i.e.
Bha mi ag iarraidh - I was wanting, I wanted
Bha mi a' tuigsinn - I was understanding, I understood
Bha mi a' smaoineachadh - I was thinking, I thought
agus, mu dheireadh
Bha mi a' fuireach (ann* an Glaschu) - I was staying/ living in Glasgow, I stayed/ lived in Glasgow
*Aon rud eile, I hadn't realised the 'ann' is often dropped. Interesting.
A-nis, tha mi 'n dòchas gu bheil mise ceart!