What is the difference between this form of past continuous and the roeddet t'in, roeddwn i'n etc?
buest ti and similar forms are generally used for actions that started and finished in some definite way the past:
- Bues i yn Llundain am fis y llynedd. - I was in London for a month last year. (Past, not habitual, defined period)
- Roeddwn i'n aros yn Llundain yn aml - I often used to stay in London. (Past, habitual, not very definite)
Good, simple explanations of the distinction are notoriously hard to come by, so I cannot pretend to have got it right here!