Achos + tense

I’ve been doing Lesson 6 of the Old Course Level 3 and it raised a few questions for me.

Firstly, the use of achos + bod / oedd. The examples given shift between tenses when the English stays the same, and I wondered whether there was a guiding principle to this. The following are examples (though it happens more often in the lesson):

“(X) didn’t go because there was no need” was translated as both

  • aeth o ddim achos oedd dim angen
  • aeth hi ddim achos bod dim angen

“(X) didn’t go because (X) didn’t want to go” as both

  • Est ti ddim achos doedd hi ddim isio mynd
  • Aeth hi ddim achos bod hi ddim isio mynd

Both constructions are obviously in widespread use or they wouldn’t be used in the lesson, but

  • is there any nuance of meaning between the two (the English is identical)?
  • Is one felt to be ‘more Welsh’ (or more correct / standard) than the other?

Secondly, please could someone explain what the ‘ei’ is doing in these sentences from the lesson:

  • Es i i weld beth oedd o’n ei fenthyg
  • Es i iweld beth oedd hi’n ei baratoi

It looks like an active verb (subject o / hi) with a masculine object/possessive pronoun (ei + SM) but what is the object referring to?

Is this commonly used, say in “Nes i ofyn beth oedd o’n ei feddwl” for example?