Gan is "by" as in agency/authorship, so llyfr gan Bethan Gwanas (a book by Bethan Gwanas).
Can it also be "with", or is this a false friend with Cornish gans in this respect? (That can express either "with" or "by".)
In other words, is "with" always gyda in Welsh or can it also be gan?
For example, "I went there with my father" (concomitative) or "I killed him with a hammer" (instrumental).
Yeah that surprised me when I first came across it in Cornish. The Welsh concomitative would be gyda in the south/formal language and efo in the north. Â is used for the instrumental meaning in formal language, but you're more likely to use gyda/efo in more informal language.
Dim problem! You seem to have knowledge of a great many languages. I'm always very impressed.
That would be ar bwys (southern), yn ymyl (northern) or maybe wrth (+ soft mutation).
Mae e ar bwys yr ysgol / Mae o yn ymyl yr ysgol (He's by/near the school now)
Mae hi wrth ddrws y tŷ (She's by/at the house door)
I swear we were taught drws nesa in school, eg Mae'r bwrdd drws nesa i'r ffenestr. Is that completely wrong?
Thinking about it, I have heard it very occasionally, but it's by no means common and sounds like something you'd say to kids: "And the table is next door to the window".
I also forgot to mention wrth ymyl (northern) and ger (formal) are options too.