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.
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Gwlad, gwlad, pleidiol wyf i´m gwlad Tra môr yn fur I'r bur hoff bau O bydded i´r heniaith barhau
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