Translation:You intend to listen to the story before the dinner.
Why isn't it "roimh an ndinnéar"?
Roimh an doesn't eclipse nouns beginning with 'd' or 't'.
It does eclipse nouns beginning with 'b', 'c', 'f', 'g', 'p'.
It also prefixes a 't' to feminine nouns beginning with s + vowel, sl, sn, sr.
In Gaeilge Uladh roimh an lenites where otherwise it would eclipse.
In English, "before the dinner" would refer to a formal event (the dinner hosted by the queen for example). In Irish would you always say "roimh an dinnéar" to mean "before dinner/before dinnertime" or is it also like a formal thing?
If ' Tá fúm' is 'I intend' would 'Bhí fúm' be 'I had intended' like 'Bhí orm' is ' I had to' agus rudaí mar sin?
According to Google translate:-
Tá fút éisteacht leis an scéal roimh an dinnéar.
"You have heard the story before dinner."
Can someone explain if this is a mistake or not?
It is a mistake!
Do not rely on Google Translate for translating Irish phrases or sentences.
Doesn't "You intend to listen...." mean the same as "You plan on listening....."? Duolongo appearantly doesn't think so. How would the latter work in Irish.
As far as elemental communication goes, you are correct. However, the sticklers would argue the verb pleanáil would be used for your idea. Yet, you know oral speech is fluid. You can give the Duolingo answer to progress through and laugh at the sticklers later as you chat your way around the Connachts!