I know this has been asked many times before and I thought that I had some grasp of it until these 2 sentences:
I didn’t understand what you said last weekend: Wnes i ddim deall
I didn’t want to wait last Friday night: Do’n i ddim yn moyn
My thinking was that last Friday night was at least as specific as last weekend so I was expecting Wnes i, or is it that the ‘understanding what they said’ is considered to be more specific than ‘wanting to wait’? If so, I don’t think that my brain is wired for these differences and it is driving me slightly insane because I seem to choose incorrectly every blinking time!
It’s because wanting is considered a state of being and verbs that are these don’t use the short form of gwneud (i.e. a preterite). “didn’t understand what you said” is a completed action, not a state, so it can use a preterite.
Thank you. I’ll copy this and try to imprint it onto my aged brain.
Don’t worry too much - although it’s frustrating to pick the wrong one, it’s only with time and exposure that you get used to which things need which, and it will sort itself out eventually as things start to ‘sound right’ (even though you might not realise why!). In the meantime, you will be understood if the wrong one creeps in, so don’t let that put you off conversations!
This is one of the things I find particularly tricky, because the difference is a bit nebulous and abstract. But I can confirm from recent experience that if you use the wrong one, people will still understand what you mean.