Roeddwn i = I was (imperfect), yr oeddwn i (literary), rôn i / ón i (spoken forms) e.g. roeddwn i'n oer ddoe = I was cold yesterday. Nes i = I did. Preterite tense of 'gwneud', gwneuthum i (literary), nes/neis i (spoken forms). Used as auxiliary verb to form other past tenses. e.g. Nes i fynd i Fangor ddoe. = I went (did go) to Bangor yesterday.

Nes i (neis = nice by the way) means 'I did' e.g. nes i fynd yno yr wythnos diwetha (I went last week) 'Roeddwn i' is more like 'i was' e.g. roeddwn i yno ddoe (I was there yesterday)

Over a year late but it's possible that someone is looking for an answer to this question or something similar. I've seen and heard them both used almost interchangeably but there is an actual grammatical difference between the two. "Roeddwn i" and all of it's conjugations (roeddet ti, roedd hi ayyb), and for that matter other verbs conjugated with the same endings, are part of the imperfect tense, the imperfect tense is a past tense which can be used to describe something over an unspecified amount of time in the past or a habitual action. Verbs such as gwnes i, gwelais i, est ti etc (usually with the ais i, aist ti, odd e ayyb endings) are the past tense. These are used usually for one off events or something which happened at a specific time, for example, "Gwelais i ffilm neithiwr" or "Aethon nhw i Sbaen ar eu gwyliau". Pob lwc!