One pattern that I’ve been thinking about for a few days now is where Welsh has an h- in place of an earlier s-. I tried looking it up in various places, but couldn’t find much on it – books and online sources just seem to say “s becomes h in early Brittonic” without being entirely clear as to whether that’s only at the beginning of words (like initial mutations), or why there seem to be so many exceptions to it, but anyway…
So many of us will already know that Halen Môn is salt from Anglesey, and that halen is saline; and Greek does the same thing (a shape with six sides is a hexagon), so the gases in headlight bulbs that ‘give birth to salts’ are halogens.
But one that I kind of knew, but had sort of forgotten, came up the other day: I saw a reference to a book on Old Irish (Hen Wyddeleg) that’s called Sengoidelc, and I went hen - sen - Oh, OK, hen as in senior (signore), senate (Senedd), senility And now I come to think of it, Tolkien describing Welsh as “the senior language of the men of Britain” was probably a deliberate, donnish pun…
And then another one came up: sedd means ‘a seat’, from the same root as English ‘sit, seat’, and I don’t know why it hasn’t changed, but it may be partly due to the influence of related words like Gorsedd where the -s- wasn’t at the start of the word. But then it turns out that it did change as well, after all, because hedd as in ‘peace’ is actually from the same word – like saying ‘settled’ in English (or, as any parent or teacher knows, “alright you lot, settle down”).
And then another one came up, when further up this thread @stephenbranley asked about dyfalu – I looked it up in the GPC and it sent me to a word hafal, meaning ‘similar’, which is apparently related both to Welsh fel as in ‘like’ and, with the usual changes, Latin similis which is where we get ‘similar’ from. (And therefore also English ‘same’.) By now it’s all getting a bit recherché again, but I might remember the word hafal.
What else can we think of? Hafren - Severn, of course; and now I’ve just looked to see if haf is related to ‘summer’ (spoiler: apparently it is) to find that the Kurdish word for haf is havîn