"Ar ôl iddi hi fynd, es i adre."
Translation:After she went, I went home.
The 'ar ôl i'..., 'rhaid i...', 'cyn i...' and similar i-dot patterns have no time/tense in themselves. The overall tense of the expression is normally implied from another part of the expression:
Ar ôl iddi hi fynd, bydda i'n gwneud y llestri - after she goes I'll do the dishes
Ar ôl iddi hi fynd, golchais i'r car - after she had gone I washed the car (or) after she went I washed the car
Ar ôl iddi hi fynd, roeddwn i'n teimlo'n drist yn aml - after she went I often used to feel sad (or) after she had gone I often used to feel sad.
I think it's because of the "Ar ôl" (After) which automatically makes the sentence past tense without having to change the verb.
Seeing more examples in the course it seems like whatever follows "Ar ôl" is always translated in the same tense as the second clause, so i guess even if the verb following Ar ôl is in the present, if the second clause is in the past tense the whole thing should be translated in the past.
Yeah that's it, ar ôl i and cyn i phrases are essentially tenseless and rely on the rest of the sentence for their tense e.g. Ar ôl iddi hi fynd, dw i'n mynd hefyd (After she goes, I'm going too), Ar ôl iddi hi fynd, es i hefyd (After she went, I went too).
I believe the first sentence "ar ôl iddi hi fynd" is subordinate to the second one "es i adre". so it must b translated in the past