I do not understand the structure of this sentence. ers = since, fi = me, fynd = go. Where is he past tense form and what is the gra mmatical role of 'i"?
The past tense is implied by the meaning of the word 'ers' = since. Literally this sentence is 'Since for me to go". "Since" refers to some past time therefore the translation is 'Since I went' or the less natural sentence 'Since I did go'.
See thread: https://www.duolingo.com/comment/14057649
Although, not sure if that helps much. But it's the same structure as rhaid i fi; in the same way that it literally means "[there is] necessity to (me, you, etc.)" instead of just using a modal verb like "must," I guess this one would be something like "[there is] since to (me, you, etc.)" instead of using a conjunction.
This statement confuses me, could it be interpreted as "Since you've been gone" ?
I assume not, although that's the closest statement that sounds right to me in English.
It's tenseless in Welsh so it could be any tense in English really: "Since you went / you were gone / you've gone / you've been gone / you'd gone / you'd been gone etc.". What you'd translate it as depends on the rest of the sentence. Let me know it that's unclear.
I think it's probably included because it's the same pattern in Welsh:
something + i + person + mutated verbnoun
Rhaid i fi fynd "I must go"
Ers i fi fynd "Since I went"
Cyn i fi fynd "Before I go/went"
Ar ôl i fi fynd "After I go/went"
Gwell i fi fynd "I'd better go"
Nes i fi fynd "Until I go/went"
Erbyn i fi fynd "By the time I go/went"