Cris Davies tweeted this last night which I think is worth a look … “Jiw - nawr dwi’n dysgu taw bwrw hen wragedd  ffyn yw’r idiom i fod!! Glaw mor drwm fel bod e’n teimlo fel ymosodiad ar rywun gwan mewn henaint! Never knew that!!”
Basically he’s explaining that he’s just learnt that the idiom for heavy rain “bwrw hen wragedd a ffyn” (raining old women and sticks) should actually be “bwrw hen wragedd â ffyn” (hitting old women with sticks) so implying it’s raining that heavy that it would hurt I guess.
All that different meaning and confusion coming from a little to bach (a/â), fascinating.
Here’s the tweet link … https://twitter.com/crisdafis/status/1144007389651361793?s=19
ETA this link from wiktionary that includes the to bach …
https://en.m.wiktionary.org/wiki/bwrw_hen_wragedd_â_ffyn
And there are others so it seems like the to bach has been optional.