Well it’s true I haven’t seen dyna yw, it’s always dyna [whatever] with nothing in between.
It’s just complicated to ask questions about tenses without verbs! 
Anyway, thanks a lot for explanations and plenty tagging!
Edit: I read your answer again and I think part of my confusion is that I can’t tell if could means something I did or something I might do.
Yes The English doesnt help. It took me 60 years to realise that could is a past tense. I always took it as future. Now our friends child who hasn’t been in school long, has a real command of the past conditional aspect. Could/should/would have etc.
You do sometimes get dyna yw, in these kind of sentences:
Dyna yw ailwylltio - That’s what rewilding is
Oedd hi’n bwrw trwy’r wythnos, ond dyna yw gwersyllu - It rained all week, but that’s camping for you
That makes perfect sense to me!
Anthony is right here - the present is included in the dyna, so any other tense requires to be expressly stated (e.g. oedd)
Yes it does make sense! But what I didn’t understand turns out is probably better asked this way:
Given that she had and still has such a kind heart, y lleiaf is something you have already done in the past or that you might do sooner or later?
Or I can use dyna oedd y lleiaf allen i gwneud for both ?
Looks like my real problem with this is that I thought -like @JohnYoung,?- it’s the least I could do is in the present.
It’s = it is = present
While for some reason it looks like it’s in the past instead!
Well it’s in two bits, isn’t it Gisella? There are two verbs there in the English. It’s the least is present, and then I could do is conditional.
Yes-which clarifies that could is conditional, thanks!
Still why not dyna y lleiaf instead of dyna oedd y lleiaf if it’s present?
P.s. sorry, I kinda wish I hadn’t asked but at this point I can’t stop until I’ve figured it out!
Oh sorry - I may have missed the exact example there.
If it’s That was the least I could do, that would indeed be Dyna oedd y lleiaf y gallwn i wneud - while That’s the least I could do is Dyna’r lleiaf y gallwn i wneud.
Not much difference in the meaning anyway, is there?
Alright! I’ve got all the options now!
Definitely not a huge difference, but my question really started from a doubt about what I was hearing (in other words:does an oedd make sense here, or it’s something else?) PLUS a doubt about the action taking place in the past or in the present/future (in the English sentence)…and how to express both cases in Welsh.
And I ended up in a big mess somehow, but glad it’s all clear now. 
But it was a very entertaining and informative mess, Gisella! 
There was no way for me to remember ymddiried, because I kept on saying ystyried instead!
I solved by challenging myself to learn a line of a song, that’s pronounced really fast. It worked! 
The only problem is that in the challenge (and in a dictionary) it’s always followed by yn, while in the songs it’s not,
Can I use both?
Or do they have a slightly different meaning/usage?
Can’t remember the examples in the challenge right now, but the one in the song goes:
Peidiwch byth ymddiried dyn efo jeans taclus 
(in case anyone’s curious)
If only I could trust you to play together without arguing! 
Rich
It’s a good question - I’ve seen it stand alone with an ‘aeth’ on the end in eg National Trust (charity)… I would have probably put an yn in your sentence uninitiated- but that would be because I don’t know any better!
( it does sound really clunky though ‘yn dyn’)
Rich
Yeah ymddiried dyn flows so.much better!
Ymddiried mewn dyn efo jîns taclus would be correct here, and I don;t have the book to hand to check the words as written, but I see on wiki quotes that the Datblygu song is quoted as ‘…ymddiried dyn…’. That, to me, sounds awfully like a (fairly horrible) word for word translation of the English ‘trust a man’.
I can’t think off hand of any examples of ymddiried without yn or mewn following, although I’m sure there are. Rich’s ‘Ymddiriedaeth’ = a trust, as in a body that is looking after something (eg the National Trust).
Thanks for the answer.
Well, word for word translation from English is certainly not the case here, cause no doubt about him as first language Welsh.
But having learned a lot of English from songs I know there are many reasons to consider…oh, wait, this is a thread about Level 3! Mae sawl reswm dros ystyried… being cautious copying sentences from songs, because they’re not written the way people speak in everyday life. Or at least, not always, and not in this case since he plays a lot with sounds and makes up words even, when he writes, and there’s the metrics and so on.
And in fact that’s why I ask here when in doubt!
I decided to try again a random challenge after…well…quite a while!
Compared to when I did the course, it’s interesting because I’m able to notice new details I had missed back then (besides of course being happy to see I can do them without using pause button, while at the time I had to write the sentences in English down cause not even pausing was enough!)
So here’s the question about challenge 7 (Level 3 South), around minute 21:
why is “would” translated as fyddai here, in general, and in following examples and not byddai?
Like the English is:
Thinking seriously would be easy for us
Even a real challenge would be easy
Next comes:
Nothing would be easy which is negative so the mutation seems to make sense to my understanding of grammar and verbs.
But shouldn’t the first two be byddai? 
In a strict grammatical way, yes, there is no apparent reason why the verb in a positive statement should undergo a soft mutation, but yet this happens quite often in natural speech.
Just think of it as if there is a positive particle mi/fe in front of the verb – which does cause a soft mutation – which is then omitted:
(Mi) fydd popeth yn iawn, t’mod. - Everything will be alright, y’know.