Thanks everyone for these tips.So much easier to thrash it out together.
It is correct to say ‘Don I ddim’ means ‘I was not’ and ‘Wnes I ddim’ means ‘I did not’. However, you are right, in that it is a bit more varied than that. Wnes i’ is used for past tense when you did something just the once like ‘I walked to the zoo’. ‘Ron i’n’ is used when you used to do something again and again in the past, like 'I used to go to the zoo when I lived I Chester. Also, you use ‘wnes i’ before verbs which involve physical activity like ‘chwarae, mynd, siarad’ etc, (when you are not using a more concise past tense form of the verbs). When you are using verbs in the past tense which are more ‘feelings’ like ‘eisiau, meddwl, teimlo’ then these are used after ‘Ron i’n, don i’n’ etc. These two types of past tense can be confusing but do seem to generally stick to the rules I have just set out. If you get it wrong, most Welsh speakers are very forgiving and I have heard that many of them get this wrong occasionally too. Hope it helps.