Indeed. "(On) Monday I have eaten strawberries" is very poor English. The completed time (on Monday) clashes with the continuing time (I have eaten). It sounds like something a German who has just started learning English might say. (No disrespect intended to German learners of English, by the way!)
I agree, I think this is phrased very awkwardly in English... but I also think it's the easiest way to signify that "wedi" is used instead of "bwytais i". I see the wording as a sort of necessary evil.
Yes, very bad English. In what context would you use this form over bwytais (which i assume means ate)?
DesertGlass, you'd use "Dw i wedi bwyta..." in the context of a time period that is still continuing. E.g. Y bore 'ma dw i wedi bwyta afal ac oren (This morning I've eaten an apple and an orange). As you suggest, if the time period were now over then you'd say "Bwytais i" (I ate): Neithiwr bwytais i oren a banana (Last night I ate an orange and a banana). Just like in English in fact!
Dydd Llun = Monday (as in "Monday is the first day of the week")
Ddydd Llun = On Monday (last Monday or next Monday, but -- in either case -- not today!)
Ar ddydd Llun = On Mondays / On a Monday (i.e, every Monday)
The literal meaning of Dw i wedi... is "I am after..."
Dw i wedi + [verbnoun] is the equivalent of the English present-perfect construction "I have* + [past participle],
Thus, Dw i wedi bwyta (lit. "I am after eating") = I have eaten.
And, yes, any time period referred to when using the construction must be an ongoing one: I've done it today (and I may yet do it again, because today's not yet over) versus I did it today (I'm looking back on the day that's now over).
So, I think you're saying that there's no real translation of those two parts incorrectly combined?
Hmm, not so sure about the "necessary", geypa. For me, it's a step too far.
(Reply to DesertGlass's comment, up-thread: "So, I think you're saying that there's no real translation of those two parts incorrectly combined?") --
Sorry if I'm being a pain about this, course devisers, ond ydw wir (yes, I am)!