"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.
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.