Well done on the first listening exercise - that’s a great sign! 
O’n i fits best with ‘I was x-ing’ - whereas ‘dw i wedi bod’ is ‘I have been’.
So ‘dwi wedi bod isio’ is okay - it means ‘I’ve been wanting’ - so you might hear people say ‘Dwi wedi bod isio gofyn rhywbeth’ and so on…
In the same way, ‘o’n i’n trio’ is ‘I was trying’, while ‘dwi wedi bod yn trio’ is ‘I have been trying’… 
Diolch Sam ac Aran.
Let’s go a touch leftfield here if I may…
Finally rain has started to fall here. Important for us as we get our water from a borehole and the taps were starting to splutter and decide water wasn’t necessary!
My elderly neighbour told me the other day that when there’s a shortage of rain she 'Dances a naked rain dance on the muck heap in the farmyard’ I had a quick go at translating into Welsh but that led me to wonder if there are any traditional sayings/idioms/incantations/spells in Welsh to encourage rain. (not that I may ever use it, just to add to my mental box of idle curiosity!)
On Gower, at least in our village, before water was privatised and Dwr Cymru decided to bring it by pipe from ??? we got ours from our own hill and never ran short even in the famous ultra-dry hot summer. ('76??). So rain dances were not needed.
Yes, our supply is from our own hill, the borehole’s 120ft deep and still running dry 
A spring fed natural pool just down the road is almost dried up for the first time I can remember. [quote=“henddraig, post:2524, topic:3153”]
never ran short even in the famous ultra-dry hot summer
[/quote]
That must have been quite something! The wisdom of forefathers when they sited their settlements!
Throw these apps away 
There was talk of one old chap who would only ever drink from the spring just below us, which had provided water for the original village before gales caused it to move. This old chap had to go into hospital and his wife took him spring water in bottles all the time he was there!
Correct [quote=“AnnaC, post:2503, topic:3153”]
THE … of THE …
A … of A …
but Anna’s house is THE house of Anna which doesn’t fit either pattern.
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In fact there is only one (two-element) pattern, and it covers all eventualities:
- All instances of THE removed, except the one (if there is one) before the last noun in the in the sequence
- All instances of OF removed
The house of Anna - tŷ Anna
The hall of the village - neuadd y pentre
a leader of an army - arweinydd byddin
the leader of the army - arweinydd y fyddin
the leader of the army of the Netherlands - arweinydd byddin yr Iseldiroedd
the leader of the army of Costa Rica - arweinydd byddin Costa Rica
Perhaps not far off according to this piece I just came across!
http://www.sacred-texts.com/neu/celt/rac/rac24.htm
“In folk-survivals the practice of rain-making is connected with sacred springs, and even now in rural France processions to shrines, usually connected with a holy well, are common in time of drought. Thus people and priest go to the fountain of Baranton in procession, singing hymns, and there pray for rain. The priest then dips his foot in the water, or throws some of it on the rocks. 1 In other cases the image of a saint is carried to a well and asperged, as divine images formerly were, or the waters are beaten or thrown into the air. 2 Another custom was that a virgin should clean out a sacred well, and formerly she had to be nude. 3 Nudity also forms part of an old ritual used in Gaul. In time of drought the girls of the village followed the youngest virgin in a state of nudity to seek the herb belinuntia. This she uprooted, and was then led to a river and there asperged by the others. In this case the asperging imitated the falling rain, and was meant to produce it automatically. While some of these rites suggest the use of magic by the folk themselves, in others the presence of the Christian priest points to the fact that, formerly, a Druid was necessary as the rain producer. In some cases the priest has inherited through long ages the rain-making or tempest-quelling powers of the pagan priesthood, and is often besought to exercise them.”
Thinking about my self-consciousness about limited vocabulary, could I say Falle dylwn i 'di paratoi yn well am y sgwrs and, if not, how would I say “Perhaps I should have prepared better for the conversation”?
Diolch @garethrking! I remember looking this up in Colloquial Welsh a while back, where the example is “the bike of the girl”, but what I apparently didn’t understand is that this pattern covers all eventualities. I remember thinking, well, what about “a girl’s bike” = “a bike of a girl” (I saw a girl’s bike by the side of the road - I knew it belonged to a girl because it was pink…).
Your set of examples is really helpful, especially the last two. I wouldn’t have understood that the “the” in “the Netherlands” counted as the last “the” for the pattern, because I would have pegged is as just part of the name. And I would have thought it was the “the army” part of “the leader of the army of Costa Rica” that was important. So thanks very much for that!
So it looks like (as a not very good example, imagining that there are multiple armies in Costa Rica with multiple leaders)
a leader of an army of Costa Rica = arweinydd byddin Costa Rica
would have to be distinguished by context from
the leader of the army of Costa Rica - arweinydd byddin Costa Rica
since they are the same.
Also, my radar being set to find examples, last night I was listening to music and picked out the line
ond pan ddaw haul y bore
from Angel by Elin Fllur. This is one where I wouldn’t have thought of “the sun of the morning” but would have just thought of morning as an adjective and said “haul bore”. I just need to keep looking out for the pattern and learning by exposure…
@RichardBuck - spot on! (I guess technically, you might use “ar gyfer” rather than “am”, but it would need a real pedant to even notice that in conversation!)
@AnnaC - the very best way to learn. Exposure until one way just feels right…
I’ve read that “ar gyfer” is the catch all for “for” and is never actually wrong (others will for better usually) so if I’m ever in doubt which one to use that’s the one I go for.
That’s rather marvelous - I’m feeling all Druidical (if thare’s such a word!), time for a ritual.
Correct!
yup, there is… derwyddol 
My word of the month, I’ll be dropping that into random sentances
When we visited Peru some years ago, at one point we were in the desert - (Atacama? - I’m not sure. Edit: possibly not. It was not far from the famous Nazca lines,and so was probably the Nazca desert) -, we saw some underground water storage caverns. I’m not sure to what extent they were natural, and to what extent excavated, but ancient (pre-Inca) people had cleverly utilised them to store water in difficult conditions. The source of the water is run-off from nearby mountains, the tops of which are often snow-capped. Many of the water caverns (cisterns?) are still functioning, and supply nearby cities.
It’s not NEVER wrong, but is safe with the main meaning of ‘for’ in the sense of ‘for the benefit of’
BUT when ‘for’ means ‘in exchange for’, or when it means ‘for a period of time’, then it IS wrong and you must use am
Nes i dalu pum punt am y llyfr 'ma
I paid five quid for this book
O’n i’n gweithio fan’na am dair blynedd
I worked there for three years
For some reason, and I don’t know where I got this idea, I thought that if it was in the past you would use ers instead of am. I misunderstood something somewhere…