"Yesterday, I have had coffee."

Translation: Ddoe, dw i wedi cael coffi.

[supermollusc]

"Yesterday I had coffee" surely?

[ibisc]

Yes, sounds wrong in both languages.

[mizinamo]

That would be more natural in my English as well.

I think the whole "wedi" section needs an overhaul to remove the sentences that have time expressions in them -- those usually take the simple past in English and, I believe, also in Welsh, which is taught later.