I'm afraid that latter does not work, Antwn. (Gwn i fod est ti i Sbaen). "Bod" cannot be followed by a conjugated verb, only by the verbnoun.
The best way for you to translate your example, Hazel is by saying:
Gwn dy fod wedi mynd i Sbaen llynedd
Rwy'n gwybod dy fod wedi mynd i Sbaen llynedd
Often, you will just have to accept that subordinate clauses generally do not have a tense and that you will realise whether it is the past or present by the tense of the main clause and the context.
Gwlad, gwlad, pleidiol wyf i´m gwlad
Tra môr yn fur
I'r bur hoff bau
O bydded i´r heniaith barhau