"I went to the museum yesterday."
Translation:Es i i'r amgueddfa ddoe.
Hmm, well, yes, technically es i'r... works, because the pronoun is usually dropped in the formal register of the language, but it is not something that you would normally expect to come across in the colloquial register as taught on Duo, etc. However, in the formal register you would normally see a different verb form being used rather than es. (I could tell you that it is euthum, but I won't, because that would only open up a whole new can of formal Welsh worms....)
Bearing in mind that the Duo course is based on a generally used, non-dialect, colloquial register of the language, you may decide to tick es i'r... if it ever appears again - and at the same time make a mental note never actually to use it!