We are told that “os byddi di wedi medru” is the same for both
“if you’ve been / you HAVE been able”. I would have thought these would be “os di wedi medru”; but that the inclusion of “byddi” in the phrase would make it mean “if you WILL have been able”.
Presumably all three meanings can apply. Have I got it wrong?
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this is a good question. Bydd here is “bod” so the been bit. “Os ti wedi medru” lacks the “been”.
I can’t think of many times I’d use this in English versus “if you’ve been able”. Examples being “I’m not sure if you will have been able to read it before the meeting” - I think I’d almost always drop the “will”. So I guess that’s why they mean the same in Welsh? 
I’m just postulating here, it’s a good question.
This is one of those situations where English and Welsh do things slightly differently; in this case it’s to do with what tense is used after “if”. Take for example a sentence like “if you go out today, please buy some bread”. The “if you go out” bit is actually referring to the future - the hypothetical going-out hasn’t happened yet, but if it does, it’ll happen in the future. So in this situation, unlike English, Welsh actually uses the future tense, saying in effect “if you WILL go out” rather than just “if you go out” (os fyddi di’n mynd allan). It sounds a bit odd in English because we don’t ever use the future tense with “if”, but it’s quite common in some other languages, including Welsh.
By contrast, if you take an example such as “if you’re happy and you know it, clap your hands”, the hypothetical happiness is right now - the speaker doesn’t know if you’re happy, but they’re wondering if you are right now. So in that case, Welsh would use the present - os wyt ti’n hapus.
But don’t worry about it too much, because even native speakers don’t do this consistently every time and it’s not something that would ever prevent you from being understood. 
Many thanks. Reading my question again I now realise that I carelessly omitted, what I had intended, namely Os di wedi " bod yn" medru. 
Thank you also Kinetic for your detailed response to my query. I must confess that I am still confused by the inclusion of “byddi” as there is no element of the “future” in the phrase “if you have been”… As you say though - not to worry! 
“If you have been there and got it sorted before 2, I’ll buy you a pint in The Bull before I jump on the train…”
Achh, English eh? …Welsh on the the other hand… 
Rich