The word 'for'

I’ve searched the internet and this forum for an answer, but I’ve come up short! The word “for”. Is there a simple explanation on when to use “am” or “i”. I know Ar gyfer is along the lines of “for the purpose of”, but “am” and “i”, I’m never 100% sure. I have instances where I naturally know why I’m using one or the other, but I couldn’t tell you why I was using it.



As far as the core meaning “for the benefit of”, ar gyfer and i are exchangeable.

When “for” is used “in exchange for” and similar meanings, the Welsh word is am
So for example “How much did you pay for that book?” or “What is the Welsh word for …?” both use am:
Faint wnest ti dalu am y llyfr’na?
Beth ydy’r gair Cymraeg am …?

There are also a few verbs that “go” with am, like “to wait/look/hope for” → aros/chwilio/gobeithio am

When “for” means “on behalf of”, it is dros:
“Do you know who is playing for Wales?” – Wyt ti’n gwybod pwy sy’n chwarae dros Gymru?