"Are you in the big one hundred metre race?"
Translation:Wyt ti yn y ras can metr fawr?
Correct. yn as a preposition does not shorten to 'n. In its two other usages it normally does shorten if it can:
- Rwyt ti yn Llundain oherwydd bydd e yn y ras yno. - preposition
- Rwyt ti'n gobeithio bydd e'n ennill. - with a verb-noun to mark an uncompleted action
- Rwyt ti'n dal, mae e'n gyflym - with an adjective or a noun
As far as I know, the yn which precedes a predicate (whether a verb, an adjective, or a noun) contracts, but the yn which is a preposition meaning "in" does not.
So I think "ti'n" would be incorrect here because you are talking about being "yn y ras" (in the race), rather than "being a race".
I understand that fawr is mutated, despite can metr is interposed between ras (feminine) and mawr , because can metr acts as a modifier or adjective (i.e. in the same way as in English 'one hundred metre' acts as a noun adjunct for 'race'), so it does not break the chain of subsequent mutations.
Instead, the conjunction a would stop further mutations, e.g. y ras fawr a cyffrous (not gyffrous).
Is this correct?