No need for a lawr/i lawr with eistedd (sitting/sitting down). Nor for lan/i fyny with sefyll (standing/standing up)
Both present and past tense are given as correct translations. Does the meaning depend on context?
Yes. i-dot patterns such as this have no tense - they depend on the context. It could even be in the future:
- Bydda i'n darllen nofel ar ôl i mi eistedd - 'I will read a novel after I sit down'
- Gwnes i ddechrau darllen nofel ar ôl i mi eistedd mewn cadair esmwyth - 'I started to read a novel after I had sat down in a comfy chair.